Five cases of malignant histiocytosis (MH) in patients being treated for acute lymphocytic leukemia (ALL) have been previously reported. Two additional cases are described here, all known cases are reviewed, and the phenomenon is discussed. The most tenable explanation for this phenomenon appears to be that ALL and MH may have been interrelated in these patients with both diseases originating from a common stem cell. with a three day history of scleral icterus. H e was noted to have hepatomegaly with a serum total bilirubin of 11.5 mg/100 ml, glutamic oxaloacetic transaminase of 1600 IU/L, lactic dehydrogenase of 630 IU/L, a n d alkaline phosphatase of 368 IU/L. A clinical diagnosis of acute viral hepatitis was made a n d he was followed as a n outpatient. O n e week later he developed a generalized pruritic maculopapular skin eruption with petechiae over his lower extremities a n d h a d a hematocrit of 40%, white blood cell ( W B C ) count of 3,9OO/cu m m with 7790 lymphocytes, a n d platelet count of 53,OOO/cu m m . O n M a r c h 15 he developed fever, chills a n d chest pain a n d was admitted to Reynolds Army Hospital. H i s temperature was 103 F with a hematocrit of 28%, WBC count of 2,300/ cu m m with 75% lymphocytes, a n d platelet count of 40,8OO/cu m m . Initial bone marrow aspiration was unsuccessful a n d he was transferred to Brooke Army Medical Center at Fort S a m Houston, Texas.Upon arrival at Brooke Army Medical Center o n M a r c h 18, the patient h a d a temperature of 104 F, generalized jaundice, hepatosplenomegaly, a n d generalized petechiae. His hematocrit was 2570, WBC count was 1,9OO/cu m m with 92% lymphocytes including occasional immature lymphcytes a n d lymphoblasts, a n d platelet count was 75,OOO/cu m m .Bone marrow aspiration a n d biopsy showed a mar-