Background. Due to the limited data available, it is not clear whether the incidence of first-onset depression varies with age in the elderly.Methods. A representative sample of individuals born 1901–2 (N = 392) was examined at the ages of 70, 75, 79, 81, 83 and 85 years by psychiatrists using a semi-structured schedule. Information on depressive episodes was also collected from self-report and examination of case records. Depression was diagnosed according to the DSM-III-R criteria.Results. The incidence of depression was 12 per 1000 person-years in men and 30 per 1000 person-years in women between the ages of 70 and 85 (sex difference P = 0·001). The incidence increased from 17 per 1000 person-years (men 8·7, women 23·2, P = 0·007) between the ages of 70 and 79 to 44 per 1000 person years (men 27·0, women 52·8, P = 0·166) between 79 and 85 (age difference: RR 2·6, P < 0·001; men RR 3·1, P = 0·036; women RR 2·3, P = 0·003) . A diagnosis of depression was associated with increased mortality and refusal rate during the 15-year follow-up. Previous episodes of depression were associated with an increased risk of further episodes. The prevalence of depression increased from 5·6% at the age of 70 to 13·0% at the age of 85. The lifetime prevalence of depression was 23% in men and 45% in women.Conclusions. Both the incidence and prevalence of depression increased with age in this longitudinally followed birth cohort, and the incidence was higher in women than in men.