This paper presents prevalence rates by race/ethnicity and age for nine psychiatric and substance use disorders found in a random sample of 728 male jail detainees. Two thirds of the sample detainees had had a disorder other than antisocial personality during their lifetimes; half of these had had an episode within 2 weeks of the interview. More than 30% currently had either a severe mental disorder or a substance use disorder. Detainees with severe mental disorders or substance use disorders were most often in jail because they had committed nonviolent crimes. Policy implications of the results are discussed.