Whereas children with Acute Leukemia are highly susceptible to infectious complications, the occurrence of acute osteomyelitis is extremely rare in these patients. The authors describe two such cases in children at onset of an acute lymphoblastic and of a myelomonocytic leukemia, respectively. In the former case, the clinical course has been characterized by the progressive involvement of several joints and bones. A citrobacter Freundii was isolated in the synovial fluid of an involved knee. This complication was successfully treated with proper antimicrobic agents and surgical toilet, while the patient was vigorously treated for her leukemia, achieving a complete remission. The latter case developed a right humerus osteomyelitis from an Enterobacter. The patient failed to respond to antibiotics, and his leukemia also turned refractory to antiblastic therapy. The difficulty in the differential diagnosis among the X-graphic aspects of leukemic, inflammatory and degenerative disease of bones are discussed by the authors. Some pathogenetic hypothesis of leukemic osteomyelitis are also presented.