Bone changes in leukemia which are detectable by palpation or radiography are not common, but when present offer some difficulty in interpretation until blood counts are made. Occasionally, also, the pain which may accompany such lesions has been diagnosed as due to osteomyelitis or rheumatic fever. Since many of these cases are associated with an aleukemic blood picture, and occur in children, the immediate diagnosis is not always obvious. The following two cases illustrate one of the several varieties of bone lesions which may be present in leukemia. Case Reports Case I: Aleukemic Lymphatic Leukemia with Gross Bone Changes: A boy of eight entered the University of California Hospital (Pediatric Service), with generalized enlargement of the lymph nodes, spleen, and liver. Blood counts showed 13,850 white cells with 93 per cent lymphocytes or unclassified cells; and again 9,900 white cells with 89 per cent of the lymphocyte series. Roentgenography (Dr. R. S. Stone) showed many lesions involving the long bones, with rarefying processes in the medulla and cortex, and lifting of the periosteum with new bone formation of a very fine lace-work type on the outside. All the bones of the arms and legs and many of the ribs were involved. The skull and pelvis showed a diffuse fine mottling. (Figs. 1 and 2.) Autopsy (29.106) showed generalized lymph node involvement and an enormous liver (2000 grams) and spleen (1320 grams), the type cell being an immature lymphocyte. The bones (femur and tibia examined) had a raised, roughened periosteum with what appeared to be almost a double cortex: an outer layer of bone separated by tumor cells and a rather friable inner layer (the true cortex) of slightly increased thickness, infiltrated with the cells. The bone marrow was replaced by tumor, parts of which were yellow and necrotic. Case II: Leukemia with Marked Bone Changes: A female infant of eleven months was first seen by Dr. C. A. Boehm of the Children's Hospital, San Francisco, on Aug. 4, 1931. She had been well until two months previous when a rash with fever developed, diagnosed as measles. Since that time there had been progressive loss of weight and strength, and the child had become pale and listless. A blood examination (Aug. 4) showed: hemoglobin 25 per cent; red cells 1,460,000, with an occasional nucleated red cell; white cells 40,300; polymorphonuclears 19 per cent; lymphocytes 80 per cent. On Sept. 20, 1931, the child was brought to the hospital because of swelling of the right leg from the knee to the ankle, accompanied by considerable pain. There were also swellings on the forehead, left brow, increasing in size. X-ray pictures (Figs. 3 and 4) showed irregular mottling of femurs, tibiae, and fibulae, with reduplication of the cortices in these bones. The appearance bore a general resemblance to a proliferating periostitis, but of much greater severity than is usually seen, and at the same time with a more regular outline of the bones. A clear line of transparent tissue was evident between the true cortex and the new-formed bone, indicating soft tissue infiltration. A previous roentgenogram of the chest had shown mediastinal masses, assumed to be lymph nodes. Cervical lymph nodes were also enlarged, but it was difficult to determine the size of the liver and spleen because of tenderness and abdominal distention. Later multiple hemorrhages occurred, generalized lymph node enlargement became apparent, and tumors developed in the skull. On Sept. 7, 1931, the blood count showed 28,900 white cells, 61 per cent of which were large or small lymphocytes, with many immature cells. The peroxidase reaction was negative, the Wassermann reaction negative, and the tuberculin skin test negative.