The possibility that a significant proportion of the patients attending a general health facility may have a mental disorder means that psychiatric conditions must be recognised and managed appropriately. This study sought to determine the prevalence of common psychiatric disorders in adult (aged 18 years and over) inpatients and outpatients seen in public, private and faith-based general hospitals, health centres and specialised clinics and units of general hospitals. Methods This was a descriptive cross-sectional study conducted in 10 health facilities. All the patients in psychiatric wards and clinics were excluded. Stratified and systematic sampling methods were used. Informed consent was obtained from all study participants. Data were collected over a 4-week period in November 2005 using various psychiatric instruments for adults. Descriptive statistics were generated using SPSS V. 11.5. Results A total of 2,nO male and female inpatients and outpatients participated in the study. In all, 42% of the subjects had symptoms of mild and severe depression. Only 114 (4.1 %) subjects had a file or working diagnosis of a psychiatric condition, which included bipolar mood disorder, schizophrenia, psychosis and depression. Concluaions The 4.1 % clinician detection rate for mental disorders means that most psychiatric disorders in general medical facilities remain undiagnosed and thus, unmanaged. This calls for improved diagnostic practices in general medical facilities in Kenya and in other similar countries.