The postpartum period can be a time when profound changes in calcium metabolism and bone mineral density (BMD) occur, particularly in association with lactation. We investigated the hypothesis that calciotrophic hormones [1,25-dihydroxyvitamin D, 25-hydroxyvitamin D, and parathyroid hormone (PTH)] are different by lactation practice or hormone status [PTH-related peptide (PTHrP), estradiol, and prolactin] and have a potential role in the bone loss and recovery associated with lactation. 1,25-Dihydroxyvitamin D, 25-hydroxyvitamin D, PTH, femoral BMD, PTHrP, prolactin, estradiol, and bone turnover markers were measured at 2 wk and at 2, 4, 6, 12, and 18 mo postparturition in 115 postpartum women aged 20-40 y (parity: 0-1). Lumbar spine BMD was measured at 2 wk and at 6, 12, and 18 mo during the postpartum period. PTH, 1,25-dihydroxyvitamin D, and 25-hydroxyvitamin D concentrations were nonlinear across the 18-mo postpartum period. Between baseline and 18 mo postparturition, PTH and 1,25-dihydroxyvitamin D concentrations did not decline, while there was a substantial decline in 25-hydroxyvitamin D concentrations. PTH, 1,25-dihydroxyvitamin D, and 25-hydroxyvitamin D concentrations did not differ by lactation practice or by PTHrP, estradiol, or prolactin status. These classic calciotrophic hormones were not associated with concentrations of bone turnover markers or changes in BMD in lactating women. In summary, patterns of change in calciotrophic hormones (PTH, 1,25-dihydroxyvitamin D, and 25-hydroxyvitamin D) in the 18-mo postpartum period appeared to be independent of PTHrP, estradiol, prolactin, or lactation status and were not associated with bone turnover markers. These data do not support the hypothesis that these three calciotrophic hormones are a central part of the calcium mobilization associated with the bone loss of lactation.