Small cell lung cancer (SCLC) frequently metastasizes to bone and bone marrow. Skeletal scintigraphy and bone marrow cytology or biopsy, are incorporated into the staging procedures to examine these organs. However, skeletal scintigraphy is not highly specific to metastases, and only one or two bone marrow sites can be examined by cytology or biopsy. We have already reported that magnetic resonance imaging (MRI) could improve the sensitivity in detecting bone marrow metastases. The result of the bone marrow MRI was an independent prognostic factor of SCLC patients [9]. In the present study, we analyzed the results of skeletal scintigraphy and bone marrow aspiration with special reference to the results of MRI examination. We also analyzed the relationship between bone marrow lesions and bone lesions. For this purpose, we visualized bone marrow metastases with MRI and determined their anatomical locations and sizes. Approximately half of bone marrow lesions stayed in bone marrow during follow-up period ranging from 57 to 154 days, whereas about half of them were accompanied by hot spots in follow-up skeletal scintigraphy, which indicates the destruction of osseous structure. Additionally, 87.5% of osteolytic changes that newly appeared in skeletal scintigraphy were preceded by adjacent bone marrow lesions. All new lesions that appeared in follow-up skeletal scintigraphy within 3 months after the initial presentation had the preceding bone marrow lesions. These results mean that almost all lesions in skeletal scintigraphy derived from bone marrow metastases. Furthermore, appreciable volume of cancer cells is present in bone marrow before osteolytic changes appear in skeletal scintigraphy.