Weight and triceps-skinfold thickness were measured until 24 mo postpartum in matched cohorts of women who breast-fed for > or = 12 mo (BF; n = 46) or < or = 3 mo (FF; n = 39). In the BF group, breast-feeding frequency and breast-milk energy output were determined every 3 mo until 18 mo. Weight loss from 1 to 12 mo postpartum was significantly greater in BF than in FF women (4.4 vs 2.4 kg, P < 0.05), due primarily to differences in weight loss from 3 to 6 mo. BF mothers had a net loss in triceps-skinfold thickness whereas FF mothers gained fat at this site (-0.4 vs 2.4 mm, P < 0.05). Breast-feeding frequency and total time breast-feeding were related to weight loss in the BF group from 6 to 12 mo. Maternal weight did not differ significantly between 12 and 24 mo in either group. We conclude that lactation enhances weight loss postpartum if breast-feeding continues for at least 6 mo.