To examine the relation between lactation and bone mineral density, we conducted a cross-sectional study among white, nonsmoking, perimenopausal women age 40-54. Three hundred fifty-two women completed a questionnaire covering medical and reproductive histories, physical activity, and diet. We measured the bone density of lumbar vertebrae 2-4 with dual photon absorptiometry, and the midradius and distal radius with single photon absorptiometry. Women with a history of lactation had 41 mg per cm2 higher lumbar bone mineral density than women with no lactation (95% confidence interval = 8-74 mg per cm2), controlling for parity, body mass, physical activity, and menopausal status. We found no important increase at mid- or distal radius.