Thirteen patients with a malignant lymphoma exhibiting the cytologic characteristics of Burkitt's lymphoma manifested marrow involvement at the time of initial diagnosis (11 patients) or later in the course of the disease (two patients). Eleven patients were 16 years of age or younger. Eight had evidence of abdominal disease, five in the ileocecal area. Despite extensive marrow involvement in most of these patients, frankly leukemic blood pictures were uncommon. Platelet counts and absolute neutrophil counts were frequently normal. Membrane surface markers were studied in four patients. The malignant cells from three exhibited mulambda surface immunoglobulin; the cells from the other manifested muk surface immunoglobulin. All of the patients had a rapid clinical course in spite of intensive chemotherapy. The median survival from detection of marrow involvement was 2.5 months.