A sixteen-year-old girl with adult-onset vitamin-D resistant osteomalacia had an elevated plasma immunoreactive parathyroid hormone level which was readily suppressed by intravenous calcium infusion. Treatment with 300,000 units of vitamin D2 normalized the high plasma immunoreactive parathyroid hormone level as well as augmented phosphaturia. Parathyroid hormone appears to play an important role in the development of vitamin-D resistant rickets in this case.