Menza et al. BMC Public Health 2010, 10:774 http://www.biomedcentral.com/1471-2458/10/774 we found that sexual risk declined in both of our study arms. However, in the case of the current study, our comparison arm was a minimal intervention control group. People who volunteer for studies may be more motivated to change their behavior than those who do not, participants may enroll during a period of high-risk activity and decrease their risk independent of interven- tion effects, and simply measuring a behavior may change a participant’s behavior or reports of that beha- vior [44]. Each of these explanations may contribute to why sexual risk in our study, as in past studies, declined in both control and CM arms. Had we not used a mini- mal intervention control group and only measured parti- cipants in the CM intervention, we might have erroneously concluded that CM reduced sexual risk. This finding emphasizes that no-treatment/minimal intervention control groups are essential to rigorous intervention evaluation. We note that the characteristics of our study popula- tion and the outcomes among CM participants were generally similar to those in a San Francisco public health program designed to reduce methamphetamine use among MSM (PROP) from December 2003 to December 2005. Our screening (14.8/month in the cur- rent study and 10.3/month in PROP) and enrollment rates (8.4/month and 7.4/month), attendance at CM vis- its (37% and 41%), and incentives earned (24% and 31% of the total possible) were similar [17]. Compared to PROP participants [18], participants in the present study were more likely to have used methamphetamine for >10 years (50% v. 38%), but were less likely to report weekly or more frequent methamphetamine use (64% v. 86%) and to be HIV-positive (55% v. 78%). More than 80% of men in both populations reported methampheta- mine use with sex and 54% of participants reported injecting methamphetamine at enrollment. These simila- rities demonstrate that our trial probably closely repli- cated what might occur in a non-research setting. Our study has several important limitations. First, we did not power the current study to detect differences between study groups based on methamphetamine use or sexual risk. Instead, this study was powered to pro- vide precise estimates of the proportion of methamphe- tamine-using MSM who report non-concordant UAI. Despite this limitation, the precision of our post- intervention period aRR rules out the potential of CM to effect a large, sustained reduction in methampheta- mine use as a stand-alone intervention, at least in the population we followed. Second, randomization did not provide us with study arms balanced with respect sev- eral pertinent variables. In particular, persons assigned to the CM arm were more likely to use methampheta- mine at baseline, and higher levels of drug use have been associated with inferior responses to CM in prior Page 11 of 13 studies [45]. It is possible that our results are subject to confounding bias stemming from imbalances in vari- ables that we did not measure. Third, we altered the CM intervention after beginning the study. These modi- fications may have not provided sufficient incentive for abstinence from methamphetamine use [20]. Finally, our measures of self-reported sexual behavior and substance use are subject to social desirability bias. Conclusions In summary, our small, randomized trial found that a 12-week CM intervention was associated with a poten- tial increase in methamphetamine use, but decreases in sexual risk that were not statistically significant. While our experience suggests that a larger, more definitive controlled trial of CM to decrease methamphetamine use and sexual risk may be feasible, our findings suggest that CM would be unlikely to effect large, sustained reductions in methamphetamine use among MSM. Acknowledgements The authors wish to thank the men who participated in this study for their invaluable contribution to HIV prevention research. The authors are indebted to David Richart, Erick Seelbach, and the rest of the staff at Lifelong AIDS Alliance for opening their organization to the study participants and staff. Don Calsyn, PhD provided helpful feedback on the manuscript. This research was supported by the National Institute on Drug Abuse (R21 DA019420-01) and Public Health–Seattle & King County. TWM was supported by the National Institutes of Health (T32 AI07140). While Public Health–Seattle & King County influenced the 3:1 randomization scheme as noted in the Methods section, the funding bodies had no other role in study design, role in the collection, analysis, and interpretation of data; in the writing of the manuscript; or, in the decision to submit the manuscript for publication. Author details 1Center for AIDS and STD, Harborview Medical Center, 325 Ninth Avenue, Seattle, WA, 98104, USA. 2Department of Epidemiology, University of Washington School of Public Health, Box 357230, Seattle, WA, 98195, USA. 3Department of Biostatistics, University of Washington School of Public Health, Box 357230, Seattle, WA, 98195, USA. 4Department of Medicine, University of Washington School of Medicine, Box 356420, Seattle, WA, 98195, USA. 5Public Health–Seattle & King County, 401 5th Ave., Suite 1300, Seattle, WA, 98104, USA. 6San Francisco Department of Public Health, 101 Grove Street, Room 408, San Francisco, CA, 94102, USA. 7Department of Family Medicine, David Geffen School of Medicine at UCLA, 50-078 Center for Health Sciences, Los Angeles, CA, 90095, USA. 8Department of Psychiatry and Biobehavioral Sciences, David Geffen School of Medicine at UCLA, 760 Westwood Plaza, Los Angeles, CA, 90095, USA. Authors’ contributions TWM participated in study design and coordination, performed the statistical analyses, and drafted the manuscript. DRJ participated in study coordination. JPH participated in the design of the study and provided statistical expertise. GNC and SS participated in the conception and design of the study. MRG conceived of the study and its design and helped draft the manuscript. All authors contributed to the interpretation of the study data, revised it critically for important intellectual content, and read and approved the final manuscript. Competing interests The authors declare that they have no competing interests. Received: 14 August 2010 Accepted: 20 December 2010 Published: 20 December 2010