Previous studies have found significant ethnic differences in the diagnosis and treatment of psychiatric disorders. This study investigated ethnic differences in referral, diagnosis and treatment of jail detainees with mental illness. Psychosocial and legal variables were assessed in 136 jail detainees admitted to a psychiatric forensic unit. We found that White patients were more frequently diagnosed with affective disorders and less frequently with psychotic disorders as compared with Black patients. No significant ethnic differences on a variety of legal variables were found. Various hypotheses to explain these results include: possible ethnic differences in rates of affective disorders in the jail population, ethnic differences in help-seeking behavior and misdiagnosis of affective disorders in Blacks by mental health clinicians.