BMC Public Health 2009, 9:340 http://www.biomedcentral.com/1471-2458/9/340 not found any studies that compare acne prevalence across populations with different diet. Our findings do not support the idea that diet influences the association between acne and mental health problems. In the study we have found an almost doubled prevalence of self-reported acne among girls with a non-Western background compared to the reference population. Also among girls, acne is more frequently reported in those who have a low income. It has also been shown previ- ously that acne can be associated with socio-demographic variables [8,9,17,49]. In the adjusted analyses, the associ- ation between the socio-demographic variables and acne is no longer significant. This highlights the importance of including relevant confounders, e.g. mental problems, in such models. Cigarette smoking and the intake of alcohol were in our study either associated with acne. Regarding smoking our results are in correspondence with a study from Denmark among girls [50], but in contrast to studies showing both increased [11] and decreased prevalence of acne in smok- ers [12]. No studies in the literature were found concern- ing acne and alcohol intake. The major limitation of this study is the cross-sectional design which limits the interpretation of the direction of the associations. Further, collecting data using a question- naire may be problematic, especially problematic con- cerning diet, but there is also the possibility of dependent misclassification [51]. The strength of the study is the non-healthcare-seeking sample with a relatively high rate of participation (80%), the use of validated instruments to measure acne and mental distress [35,36] and the inclusion of possible confounders in the multivariate analyses which are lacking in similar studies [15- 17,21,22]. Conclusion In this study a consistent association between acne and mental problems has been shown for both genders in a model which also includes diet, lifestyle factors and socio- demographic variables. The findings do not support the hypothesis that diet may have a confounding effect on the association between acne and mental problems. This study points to mental problems as an important factor in acne, but the causal relationship remains elusive. There- fore we would suggest that measuring psychological parameters should be included in future studies on acne, especially in studies with a longitudinal design. Acne has been shown to have high heritability in twin studies [52]. However, it is important to be aware that this does not automatically imply that environmental factors have no influence. If the whole population is exposed to the same risk factors, genetic factors will emerge as a strong deter- minant of disease. The fact that acne is rarely seen in some non-Western societies [24] should encourage a further search for risk factors. This study shows that both dietary factors and mental health problems are candidates as causative factors in the development of acne among ado- lescents in a Western country. Competing interests The authors declare that they have no competing interests. Authors' contributions JAH carried out the statistical analyses and drafted the manuscript. FD took part in planning the study, designed part of the questionnaire and commented on the drafts. MT took part in the statistical analyses and commented on the draft. EB took part in planning the study, participated in its design and coordination, and commented on the draft. LL commented on the statistical analyses and the draft. All authors read and approved the final manuscript. 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