Menza et al. BMC Public Health 2010, 10:774 http://www.biomedcentral.com/1471-2458/10/774 from substance use) and withholding incentives when the goal is not met. Scientists have used CM to success- fully promote an array of behavioral goals in diverse populations [12-14]. In a randomized trial that enrolled MSM seeking outpatient drug treatment, CM and cogni- tive behavioral therapies resulted in similar reductions in methamphetamine use and sexual risk [15]. The results of that trial, as well as data indicating high levels of interest in CM [16], prompted the San Francisco Department of Public Health to implement the Positive Reinforcement Opportunities Project (PROP), a stand- alone CM intervention among non-treatment-seeking MSM [17,18]. Despite evidence supporting the potential effectiveness of CM, the intervention’s effects on methamphetamine use and sexual risk have not been tested using a mini- mal intervention control. We conducted a randomized controlled trial of CM to reduce methamphetamine use and sexual risk among MSM outside of drug treatment. The goals of the study were to assess the feasibility of conducting a larger, more definitive trial of CM; to define the frequency of behavioral outcomes to power such a trial; and, to compute preliminary estimates of CM’s effectiveness. Methods Study population We enrolled MSM from Seattle, WA. Between June 2007 and August 2008, participants were recruited through referrals from an STI clinic; an HIV clinic; community-based organizations (CBOs); printed media; radio; community outreach; and, referral by peers parti- cipating in the study. This study was initially designed to inform the design of a trial to test whether CM could prevent HIV acquisi- tion among HIV-negative men who use methampheta- mine. Therefore, from June 2007 to November 2007, we enrolled only HIV-negative men. Because of slow enroll- ment of HIV-negative men, in November 2007, we opened the study to HIV-positive men and revised the study objective to test whether a CM intervention could reduce HIV acquisition and transmission risk. Several studies illustrate that a considerable proportion of HIV- positive men use methamphetamine and that metham- phetamine facilitates HIV transmission behaviors [9]. Additional inclusion criteria were: age ≥18 years; willing- ness to be randomized and provide locator information; and, no plans to move from the study catchment area within 6 months of enrollment. Behavioral eligibility cri- teria were ≥1 episodes of anal sex and ≥2 episodes of methamphetamine use in the month prior to screening. We did not require a methamphetamine-positive urine sample for enrollment as we thought this requirement would limit the utility of CM as a public health Page 2 of 13 intervention in practice. We excluded participants who reported a mutually monogamous relationship with a partner of the same HIV status lasting ≥2 years and men who expressly asked for detoxification, counseling, or drug rehabilitation services. HIV status was the only eligibility criterion modified during the course of the study. Study intervention and design The study intervention was a 12-week CM program, adapted from previous studies, in which vouchers of escalating value were offered for consecutive urine sam- ples that did not contain methamphetamine or crack or cocaine (herein referred to as cocaine) metabolites [15]. Initially, the intervention consisted of thrice-weekly drop-in urine testing visits for a total of 36 visits. Vou- chers started at $2.50 for the first stimulant-free sample and increased by $1.25 for every consecutive stimulant- free sample thereafter up to a maximum of $10.00. Par- ticipants submitting three stimulant-free urine samples in a row earned a $10.00 bonus. The maximum payout for this program was $453.75, similar to the payout in other programs [17,18]. When participants submitted a stimulant-containing sample, or missed a visit, no vou- chers were issued and the value of the voucher for the subsequent stimulant-free sample was reset to $2.50. If a participant submitted a week of stimulant-free samples after submitting a sample containing stimulant metabo- lites, he returned to the voucher value prior to the sti- mulant-containing sample ("rapid reset”) [19]. Vouchers were redeemable immediately upon accrual for pre-paid gift cards and goods and services; we never offered cash. All CM participants signed a contract delineating the expectations of the CM program [20]. Study personnel administering the CM intervention followed a simple, scripted protocol for the reporting of results of urine testing and administering vouchers. Such a protocol was used to avoid the provision of counseling around the results of the urine testing. The protocol, which required no formal training, was developed by one of us (SS) who has extensive expertise in delivering CM interventions. All seven participants enrolled in the study while the above procedures were in place reported difficulty adhering to the intervention schedule, and only two attended ≥12 of their 36 visits. In response, in Septem- ber 2007, we reduced the number of weekly urine test- ing visits to two (24 visits over the 12-week intervention) and increased the value of vouchers for the first stimulant-free urine sample to $7.50; other stu- dies have employed a similar schedule [21]. As before, these vouchers increased by $1.25 for each consecutive stimulant-free sample to a maximum of $10.00. Addi- tionally, we gave participants a $20.00 bonus for two