Of 213 children with various solid tumors, 493 bone marrow aspirates were examined. Malignant cells were found in bone marrows of 49 of 90 patients with neuroblastoma, 5 of 25 patients with rhabdomyosarcoma, 3 of 13 patients with retinoblastoma, 1 of 13 patients with miscellaneous sarcomas, and 2 of 5 patients with ganglioneuroblastoma. The PAS reaction was helpful in the recognition of rhabdomyosarcoma cells. Tumor cells in the marrows of four of five patients with rhabdomyosarcoma were PAS positive. Bone marrow examinations were useful in selecting an appropriate treatment program, in assessing prognosis, and in determining response to therapy.