General population data are presented on the prevalence and correlates of comorbidity between DSM-III-R major depressive disorder (MDD) and other DSM-III-R disorders. The data come from the US National Comorbidity Survey, a large general population survey of persons aged 15-54 years in the non-institutionalised civilian population. Diagnoses are based on a modified version of the Composite International Diagnostic Interview (CIDI). The analysis shows that most cases of lifetime MDD are secondary. In the sense that they occur in people with a prior history of another DSM-III-R disorder. Anxiety disorders are the most common primary disorders. The time-lagged effects of most primary disorders on the risk of subsequent MDD continue for many years without change in magnitude. Secondary MDD is, in general, more persistent and severe than pure or primary MDD. This has special public health significance because lifetime prevalence of secondary MDD has increased in recent cohorts, while the prevalence of pure and primary depression has remained unchanged.