A longitudinal investigation of psychiatric epidemiology in a general population (the Stirling County study) has indicated that the incidence of depression and anxiety disorders is low relative to prevalence, because these disorders have long durations. In an average year approximately nine adults among 1,000 experience a first-ever episode of one of these disorders. Incident cases over the course of a 16-year follow-up were more likely to have had premonitory symptoms than to have been asymptomatic at the beginning of the study. Among the relatively small number of people who exhibited the clearest prodromal manifestations, incidence was 20 per 1,000 annually. It might be possible to intervene before such disorders become fully formed and persistent if the precursors are given attention.