Results on the age/sex specific prevalence of DSM-III affective disorders from the NIMH Epidemiologic Catchment Area Study (ECA), a probability sample of over 18,000 adults from five United States communities, are presented. The cross-site means for bipolar disorder ranged from 0.7/100 (2 weeks) to 1.2/100 (lifetime), with a mean age of onset of 21 years and no sex difference in rates. The cross-site means for major depression ranged from 1.5/100 (2 weeks) to 4.4/100 (lifetime), with a mean age onset of 27 years and higher rates in women. The cross-site means for dysthymia, a chronic condition, was 3.1/100 with a higher rate in women. There was reasonable consistency in prevalence rates among sites. The implications of these findings for understanding psychopathology are discussed.