Although several nutrients and foods are suggested to be preventive against postpartum depression, all previous studies have primarily focused on single nutrients or foods. In contrast, studies on dietary patterns, namely the measurement of overall diet by considering the cumulative effects of nutrient, may provide new insights into the influence of diet on postpartum depression. We prospectively examined the association between dietary patterns during pregnancy and the risk of postpartum depression among 865 Japanese women. Diet was assessed with a validated, self-administered diet history questionnaire. Dietary patterns from thirty-three predefined food groups (energy-adjusted food (g/d)) were extracted by factor analysis. Postpartum depression was defined as present when the subjects had an Edinburgh Postnatal Depression Scale score of >= 9 at 2-9 months postpartum. A total of 121 women (14.0%) were classified as having postpartum depression. Three dietary patterns were identified: 'Healthy', 'Western' and 'Japanese' patterns. After adjustment for potential confounders, neither the 'Healthy' nor the 'Japanese' pattern was related to the risk of postpartum depression. Compared with the first quartile of the 'Western' pattern, only the second quartile was independently related to a decreased risk of postpartum depression (multivariate OR 0.52, 95% CI 0.30, 0.93), although no evident exposure-response associations were observed (P for trend=0.36). The present study failed to substantiate clear associations between dietary patterns and the risk of postpartum depression. Further studies with more accurate measurements are warranted to confirm the relationship between dietary patterns and the risk of postpartum depression.