Two hundred six technetium-99m sulfur colloid bone marrow scans in 110 pediatric patients were reviewed. The normal distribution of sulfur colloid in the lower extremities in various age groups was established. There was progressive loss of uptake with increasing age from less than two years to greater than ten years. Tumor replacement was seen as regions of decreased radioactivity, and the extent of the scan defect paralleled the response of the disease to therapy. Both chemotherapy and irradiation resulted in an extension of the Tc-99m SC to peripheral marrow sites. In irradiated areas, marrow scan defects were demonstrated and generally recovered normal activity by six months after the completion of therapy. Marrow scan abnormalities caused by tumor replacement were present in four patients despite normal bone scans and radiographs. Ultimate confirmation of tumor involvement was by needle aspiration or biopsy. Persistent marrow defects were seen in two patients with neuroblastoma who had remission of their disease; biopsy revealed myelofibrosis. Technetium-99m sulfur colloid bone marrow scanning is a sensitive monitor of altered marrow activity associated with pediatric hematologic or oncologic diseases.