We describe the changes in calcium homeostasis seen in a hypoparathyroid woman during the third trimester and with lactation following her second pregnancy. During lactation her need for supplemental calcium and calcitriol abated, and in fact she was transiently hypercalcemic and hypophosphatemic. This change was associated with a rise of serum parathyroid hormone-related peptide (PTHrP) released systemically during lactation. This is the first documentation of the time course of serum PTHrP levels from the late third trimester throughout lactation in a hypoparathyroid woman. In this context PTHrP may have sufficient biological activity to compensate for parathyroid hormone deficiency.