There is little information on the effect of lactation on maternal mineral and calcitropic hormone status. Therefore, we prospectively compared 26 lactating women with 32 nonlactating postpartum controls over the first year postpartum. Nineteen of the 26 women breast-fed their infants for fewer than 12 months and seven breast-fed for at least 12 months. During the first 6 months postpartum, serum phosphorus and parathyroid hormone (PTH) concentrations decreased with increasing time (P=. 04 and P=. 003, respectively) and were higher in lactating compared with nonlactating women (P<. 001 and P=. 06, respectively). Mean serum phosphorus concentrations at 1, 3, and 6 months postpartum were 4.45, 4.75, and 4.34 mg/dL, respectively, in lactating women, versus 4.01, 3.64, and 3.44 mg/dL in controls. Mean PTH concentrations were 1.58, 1.48, and 1.36 ng/mL in lactating women, compared with 1.45, 1.20, and 1.16 ng/mL, respectively, in controls. At 12 months, women who were weaning had significantly higher mean serum calcium (10.11 mg/dL) and magnesium (2.36 mg/dL) concentrations than those who had weaned (8.79 and 2.03 mg/dL, respectively) or who had never lactated (8.90 and 1.95 mg/dL, respectively). Serum phosphorus, PTH, and 1, 25-dihydroxyvitamin D were similar among women who were weaning (4.02 mg/dL, 1.46 ng/mL, and 54 pg/mL, respectively) and those who had weaned (3.94 mg/dL, 1.68 ng/mL, and 55 pg/mL), and were significantly higher than concentrations in women who had never lactated (3.25 mg/dL, 0.92 ng/mL, and 39 pg/mL). Our findings during lactation and the persistent differences observed during and after weaning are consistent with bone mobilization during lactation and a recovery of bone mass during and after weaning