Prior study has provided data suggesting that interferon gamma 1b improves bone resorption, yielding an increse in bone marrow space; improves white cell super-oxide production, and reduces the trend toward progressive narrowing of the cranial nerve foramina. This study proposes to determine if interferon gamma 1b therapy combined with calcitriol can improve bone turnover, marrow function, and the size of cranial nerve foramina in severe osteopetrosis to a greater extent than calcitriol alone through a randomized, controlled, open-label trial.