Myeloma and lymphoma are malignant round cell lesions. Round cell lesions are characterized by growth of undifferentiated round cells without matrix production. Myeloma is the most common primary malignant bone tumor. It is originated from bone marrow and seen mostly in adults. Myeloma should be included in differential diagnosis of all patients after 40 years old with bone tumors. Lymphoma can affect the bone primarily or secondarily. Secondary bone lymphoma which is associated with nodal and visceral involvement is more common than primary lymphoma of bone. Ewing sarcoma and other malignant round cell tumors and osteomyelitis should be included in the differential diagnosis of bone lymphoma. Acute leukemia is the most common cancer of childhood and frequently associated with bone and joint involvement. Leukemic infiltration should be suspected in a child who presented with bone pain and associated symptoms such as cytopenia, fever, bleeding manifestations, hepatosplenomegaly and lympadenopathy.