INTRODUCTION: The purpose of this study was to examine the risk factors for postnatal depression (PND) using longitudinal data in a representative sample of Australian women. METHODS: Mailed survey data collected from the youngest cohort of the Australian Longitudinal Study on Women's Health were analyzed. Of the women in this cohort, 2451 had a child in the 4 years preceding survey 4 in 2006, and those who reported being diagnosed or treated for PND at survey 4 (n = 252) were compared to those who had no report of PND at survey 4 (n = 2324) in order to identify risk factors for PND. RESULTS: Women with a history of depression (from survey 2 in 2000 and from survey 3 in 2003) were more likely to report postnatal depression (odds ratio [OR] 2.10; 95% confidence interval [CI], 1.39-3.18 and OR 2.15; 95% CI, 1.37-3.35, respectively). Contrary to previous research, demographic factors were not significantly related to PND. Women who rated their affectionate support and positive social interaction as being available some of the time were significantly more likely to experience PND (OR 2.37; 95% CI, 1.24-4.53) than those who rated this type of support as being available all of the time. DISCUSSION: While previous mental health problems and a history of stressful life events were found to be significant risk factors for PND, the results of this study show that women with PND are also lacking some aspects of social support around the time of the birth of their children. Implications for treatment and policy are discussed.