he purpose of this study was to estimate the prevalence of anabolic androgenic steroid (AAS) use and offers to use among gym users in Stockholm County (Sweden), and to conduct a comparison of concordance in estimates of AAS and supplements at gyms between two data collection methods. A questionnaire was distributed to members at 36 training facilities and 1,752 gym users participated in the study. An observation study was conducted as covert participant observations at 64 gyms. According to the questionnaire, 3.9% of men reported life time use of AAS, 1.4% use during the past 12 months and 0.4% AAS use during past 30 days. Not only were there similar patterns found in the two methods, i.e., similar age and gender distributions for AAS use, but analyses of concordance showed that gyms with a higher prevalence of self-reported AAS-use and supplement use (questionnaire) showed a significantly higher proportion of observer-assessed AAS users. Analyses of individual predictors showed that AAS users were almost always young men, regular weight trainers and more often users of drugs and nutritional supplements. The higher prevalence of AAS use among gym users than in the general population makes the former an appropriate target group for AAS prevention. The connection between supplements, drugs and AAS use suggests that effective AAS prevention need to focus on several risk factors for AAS use. The clear resemblance in estimates between the observation and questionnaire data strengthen the credibility of the two methods.