A randomized, placebo-controlled calcium supplementation study has investigated the benefits of increased calcium intake during 6 months of full breast-feeding and during the weaning period for lactating women with a dietary calcium intake below 800 mg/day, compared with nonlactating women who had recently given birth. The calcium supplement of 1000 mg/day had no impact on breast milk calcium concentration or on lactation-associated bone mineral changes in the lumbar spine, radius, or total body. Calcium supplementation produced a modest increase in spine bone mineral density in both lactating and nonlactating women, but the potential significance of this effect is unclear. The results of this study support and extend the findings from three previous supplementation studies and suggest that women do not need to consume extra calcium during breast-feeding.