To the Editor: Key et al. (Feb. 16 issue)1 describe an infant with malignant osteopetrosis who had a small clinical response to highdose calcitriol (32 μg per day). A morphologic and functional change in osteoclasts was noted. We have administered high-dose calcitriol (24 μg per day) to two infants with malignant osteopetrosis. The first presented at the age of two months with mild frontal bossing, cranial-nerve dysfunction, massive hepatosplenomegaly, pallor, and developmental delay. Biochemical aberrations before therapy included elevated serum levels of parathyroid hormone, acid phosphatase, and 1,25-dihydroxyvitamin D and decreased levels of serum calcium and phosphorus. The second infant,.