The effect of lactation on weight retention was investigated longitudinally, with data collected at 0.5, 2, 4, 6, 12, and 18 mo after parturition in 110 women aged 20-40 y who had been nulliparous or primiparous. At each evaluation women were categorized as fully breast-feeding, partly breast-feeding, or bottle-feeding including infants weaned to a bottle (bottle feeding/weaned). Postpartum weight retention was calculated by subtracting weight before pregnancy from weight at each evaluation. Lactation practices were found to be significantly associated (P < 0.05) with postpartum weight retention by longitudinal regression analysis. Women who bottle-fed their infants retained more weight over time than women who breast-fed their infants. Significantly slower rates of weight loss were observed when women ceased breast-feeding or switched from fully to partly breast-feeding. Weight retention over time was greater in women who were older, unmarried, or had greater weight gain during pregnancy (P < 0.05). A pattern of weight gain rather than weight loss was observed in unmarried women. Our findings suggest that lactation influences the pattern of postpartum weight retention; however, the effect of lactation on weight retention was sufficiently limited to warrant minimal emphasis on lactation as a means of minimizing postpartum weight retention.