Painful bone disease, characterized by patchy osteomalacia and inactive bone, can develop in patients treated with total parenteral nutrition for more than 3 months. Serum levels of 1α, 25-dihydroxyvitamin D (1α, 25(OH)2D), 24,25-dihydroxyvitamin D and 25-hydroxyvitamin D were measured in seven adults and five children treated with parenteral nutrition for 9 to 60 months. Serum levels of 1α,25(OH)2D were markedly reduced, while levels of 25-hydroxyvitamin D and 24,25-dihydroxyvitamin D were normal. Serum calcium and phosphorus levels were normal or slightly increased, and immunoreactive parathyroid hormone levels were normal or low. Renal function was normal or minimally reduced. Skeletal symptoms disappeared and serum 1α,25(OH)2D levels rose to normal in one patient when nutrient infusions were discontinued for 6 weeks. Removal of calcium from the nutrient solution for 2 to 4 days was associated with no change in serum 1α,25(OH)2D in two patients. The cause of the reduction in serum levels of 1α,25(OH)2D and its role in the pathogenesis of bone disease in these patients remain uncertain.