This study prospectively investigated the factors underlying the maintenance and persistence of postnatal depression beyond the first year after birth. One hundred primiparous women who were admitted to a parentcraft hospital for a week were assessed after discharge at 4 and 12 months postpartum. Various measures of mood, interpersonal relationships and defence styles were administered at 4 months and the relation between these measures and clinically elevated symptoms of depression at 12 months was examined. At 12 months, 30% of all mothers and 60% of those diagnosed depressed at 4 months continued to report clinically significant levels of depressive symptomatology. The strongest predictor of depression at 12 months was severity of symptoms at 4 months, and women from a non-English speaking background were significantly more likely to remain depressed. Reports of low maternal care in childhood, marital dissatisfaction at 4 months, an attachment style characterised by anxiety over relationships and immature defence styles were significant predictors of clinically elevated depression scale scores at 12 months. Furthermore, an insecure attachment style was shown to mediate the effect of low maternal care in childhood, while other cognitive and interpersonal factors appeared to contribute additively in maintaining depressive symptoms. Self-report measures were used to measure insecure attachment styles and depression at 12 months. Findings demonstrate that both childhood and concurrent relationship difficulties contribute to the maintenance of postpartum depression. Interventions for persistent depression need to address relationship difficulties as well as depressive symptomatology.