The objectives of this study were to determine the risk factors for retained placenta by evaluating several reproductive factors in individual cows, and to determine the effects of retained placenta on the occurrence of postpartum diseases and subsequent reproductive performance in dairy herds. The health status, cow parity, calving, and breeding dates were recorded from 805 calvings in nine dairy herds from October 2000 to March 2004. We used logistic regression to evaluate the effects of these factors on the incidence of retained placenta. A stepwise procedure, used to obtain the appropriate model with alpha = 0.05, revealed that abnormal partus and gestation length were important risk factors for retained placenta. The occurrence of endometritis and metabolic disorders was greater (p<0.01) in the group with retained placentas than in the control group. The mean intervals from calving to first service and conception were prolonged (p<0.05) by 7 and 18 days, respectively, in the group with retained placentas compared to the control group. These results suggest that abnormal partus and gestation length are strongly correlated with the development of retained placenta, which increases the occurrence of postpartum diseases and subsequently decreases reproductive performance in dairy herds.