Background. The aim of this study was to determine the prevalence of psychiatric morbidity in selected semiurban primary care centers in Lagos Island, Nigeria using the screening tool GHQ-12 and the ICD 10 mental disorders checklist (primary care) (ICD 10 PC). Methods. In this multistage cross-sectional study, 400 participants were recruited by using proportional sampling of 17,787 attendees. Results. Of all respondents, 45.8% scored positive on GHQ-12. The most prevalent "any ICD 10 disorder" was unexplained somatic disorder (57.5%), while 2.0 to 7.8% of the respondents reported varying levels of disablement. Younger age (P < 0.001) and being widowed (P = 0.03) were significantly associated with high GHQ scores while younger age (P < 0.001) and male gender (P = 0.04) were significantly associated with "any ICD 10 disorder". Conclusion. These findings are a useful guide to the probable prevalence of psychiatric morbidity in primary care in Nigeria and in the design of appropriate interventions.