SummaryReport of five cases of leukemia in which pains in the bones or joints were at one time the main symptom of the disease. The patients were three girls, 8, 9 and 10 years old, and two boys, 9 and 15 years old, living in or near Brussels.A survey of the literature shows that these pains occur also in adults, but not so frequently as during the growth-period. The clinical picture with joint-pains and fever can be so misleading that acute rheumatism is often first diagnosed. Other cases have been mistaken for acute osteomyelitis or for Still’s disease. What frequently increases the diagnostic difficulties is the fact that the leukemias, particularly in children, are often not accompanied by an increased total white count and that the morphological characteristics of the leukemia-cells existing in some of these cases are not always recognised by the untrained observer. The pain can persist for a long period of time; frequently it disappears spontaneously after 4 or 5 days and recurs after a period of variable duration. The affected bones or joints may be swollen.Post-mortem examinations of patients with leukemia may reveal in a certain number of cases osteoplastic and osteoclastic bone changes : osteodystrophy, osteoporosis (decalcification), osteosclerosis, osteolysis, osteonecrosis, periostosis or periosteal thickening (the leukemic infiltration of the bone-marrow breaks through the cortex and lifts the periosteum, which can then build a narrow layer of bone parallel to the shaft). In children the lesions have a somewhat peculiar aspect, due to growth-disturbances; they are found frequently in the meta-physes. The joint manifestations appear to be due mostly to neighboring bone lesions, for the joint structures themselves are rarely infiltrated.The X-ray findings are :1°Osteoporosis : localized or generalized.2°A narrow transverse zone of diminished density in the metaphysis.3°Periosteal thickening and layering.4°Areas of bone destruction, particularly at the ends of the diaphyses, which make the bone look as if it had been nibbled by mice. These are frequent in the small bones of the hand.5°Osteosclerosis.None of these lesions is pathognostic for leukemia : osteoporosis and osteosclerosis are quite uncharacteristic; periosteal thickening is found among other conditions in congenital syphilis and in pulmonary osteoarthropathy; the narrow transverse zone of diminished density is seen also in the osteochondritis of congenital syphilis. According to Doub and Hartman however«the periosteal changes in conjunction with the peculiar moth-eaten areas of erosion at the ends of the diaphyses... are changes that are rather uncommon in other diseases. When these are associated with the pinpoint areas of decalcification in the flat bones, one should suspect some myeloid or lymphoid neoplastic condition». Sympathetic neuroblastoma however can give the same association of roentgenological aspects.Several authors have shown that these X-ray signs may disappear.