BACKGROUND Evidence from developed industrialized countries suggests poor uptake of mental health services. No data exist in developing resource-constrained countries about met and unmet need for mental health service in the community. METHODS A four-stage stratified probability sample of households was studied in the Yoruba-speaking part of Nigeria (population, approximately 25 million people or 22% of the Nigerian national population). Face-to-face interviews were conducted with persons 18 years old and above (n=4,984) using the World Mental Health version of the Composite International Diagnostic Interview. We determined the proportions of respondents with 12-month Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition (DSM-IV) anxiety, mood, or substance use disorder who had received any mental health treatment and the correlates of treatment receipt. RESULTS Only 9.0% of those with any 12-month DSM-IV disorder had received treatment. While 11% of those with a mood disorder had received some treatment, none of those with substance use disorders had used a mental health service. Most treatments were received from general medical settings, with only about 1% of those with DSM-IV disorders receiving specialist mental health service. Surprisingly, complementary or alternative health providers were also consulted by only about 4% of those with mental disorders, although a much higher proportion of 57% of those with no DSM-IV disorders but who nevertheless received mental health treatment did so from such providers. Irrespective of the disorders or the sector where treatment was received, virtually no treatment was adjudged minimally adequate. CONCLUSIONS There is a striking level of unmet need for mental health service in the community in this developing country setting. While inadequacy of the formal public health sector may be partly responsible for this observation, there is the likelihood that receipt of treatment for mental health problems may also be hampered by the public's poor knowledge of the nature of the disorders and by stigma.