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Evidence from ecological studies and from studies of and sexually transmitted disease (STD) patients in sub-Saharan Africa suggests that there is a protective effect of male circumcision against HIV infection. There are, however, few population-based studies that have controlled adequately for potential confounding factors.Data from five population-based studies in north-western Tanzania were used to investigate the association between male circumcision and the risk of HIV infection and STD. The effects of circumcision on HIV prevalence, syphilis (positive Treponema pallidum haemagglutination; TPHA) and self-reported STD were analysed, controlling for a range of demographic and sociocultural variables, and indicators of sexual behaviour.In north-western Tanzania, circumcision was previously restricted to Muslims and specific ethnic groups, but is now more widespread, particularly in urban areas and among more educated men. Assessment of the reliability and validity of self-reported circumcision status showed that these data could be considered fairly accurate, although there was some tendency for circumcision to be over-reported. On univariate analysis, circumcision status was unrelated to HIV prevalence in most studies. After controlling for confounding variables, however, there was a modest but significant reduction of the HIV prevalence among circumcised men [odds ratio (OR), 0.62; 95% confidence interval (CI), 0.48–0.81]. This effect appeared stronger in urban areas (OR, 0.46; 95% CI, 0.32–0.68) and roadside villages (OR, 0.65; 95% CI, 0.42–1.01) than in rural areas and islands (OR, 1.00 and 1.01 respectively). There was no association between circumcision status and syphilis serology (TPHA), but there was a positive association between circumcision and self-reported STD, although this was not significant after adjustment for confounding variables.Male circumcision has a protective effect against HIV infection in this population, which may be stronger in urban areas and roadside settlements than in the rural areas. Ethnic group and religious denomination are no longer the sole determinants of male circumcision.