OBJECTIVE To test the hypothesis that bone mineral content (BMC) and density (BMD) are lost during lactation and regained within 6 months after weaning. METHODS Two cohorts of women, defined by time postpartum, were enrolled into the study; each cohort was followed for 6 months. Women in the lactation cohort (65 lactating women and 48 nonlactating postpartum controls) were enrolled at 2 weeks postpartum. Women in the weaning cohort (40 lactating and 43 nonlactating postpartum controls) were enrolled at 4-6 months postpartum. Lactating women enrolled in the weaning cohort had been fully breast-feeding at enrollment and weaned within 2 months of enrollment. Bone mineral content of the total body and BMD of the lumbar spine and distal radius were measured by dual-energy x-ray absorptiometry. RESULTS Lactating women lost significantly more bone in the total body (-2.8 versus -1.7%) and lumbar spine (-3.9 versus 1.5%) than did nonlactating women during the first 6 months postpartum. There was no effect of lactation on bone changes at the distal radius. After weaning, lactating women gained significantly more bone in the lumbar spine than did nonlactating women (5.5 versus 1.8%). Earlier resumption of menses was associated with a smaller loss of bone during lactation and a greater increase of bone after weaning. CONCLUSIONS Women lose bone during lactation but gain bone after weaning. Thus, lactation may not result in net bone loss.