Background Large-scale community studies of the prevalence of mental disorders using standardised assessment tools are rare in sub-Saharan Africa.

Aims To conduct such a study.

Method Multistage stratified clustered sampling of households in the Yoruba-speaking parts of Nigeria. Face-to-face interviews used the World Mental Health version of the Composite International Diagnostic Interview (WMH–CIDI).

Results Of the 4984 people interviewed (response rate 79.9%), 12.1% had a lifetime rate of at least one DSM–IV disorder and 5.8% had 12-month disorders. Anxiety disorders were the most common (5.7% lifetime, 4.1% 12-month rates) but virtually no generalised anxiety or post-traumatic stress disorder were identified. Of the 23% who had seriously disabling disorders, only about 8% had received treatment in the preceding 12 months. Treatment was mostly provided by general medical practitioners; only a few were treated by alternative practitioners such as traditional healers.

Conclusions The observed low rates seem to reflect demographic and ascertainment factors. There was a large burden of unmet need for care among people with serious disorders.