OBJECTIVE - To determine the prevalence of depression in adult diabetic populations through a comprehensive literature review and to critically evaluate the methods and findings of such studies from an epidemiological perspective. RESEARCH DESIGN AND METHODS - A systematic review of the scientific literature revealed a total of 20 studies, 14 of which had been conducted since 1988. Nine of the studies were controlled investigations, whereas the remaining 11 studies did not contain comparison groups. The studies included both treatment and community samples. RESULTS - The range of the prevalence of current depression obtained from structured diagnostic interviews in diabetic samples was 8.5-27.3% (X̄ = 14.0%) in controlled studies and 11.0-19.9% (X̄ = 15.4%) in uncontrolled studies. These rates are at least three times the prevalence of major depressive disorder found in the general adult population of the U.S. Investigations using depression symptom scales corroborated these findings, as the range of clinically significant depression symptomatology in diabetic samples was 21.8-60.0% (X̄ = 32.4%) in controlled studies and 10.0-28.0% (X̄ = 19.6%) in uncontrolled studies. CONCLUSIONS - An increased prevalence of depression in diabetes relative to the general population is highly suggested by the literature, but biases and methodological problems commonly encountered in prevalence studies may interfere with the strength of this conclusion. An increased prevalence of depression in diabetes relative to other somatic illnesses remains unproven. The pervasive impact of depression on quality of life and its potential negative effect on diabetes management warrant recognition and treatment of the affective disorder in diabetic individuals.