A lot of hematologists are often faced with the difficulty of diagnosing bone marrow micrometastasis of carcinoma cells. We employed a new flow cytometric immunophenotyping by a combination of CD45 with three neuroendocrine markers: CD56, microtubule-associated protein-2 and synaptophysin, and successfully detected micrometastatic tumor cells in the bone marrow of a 61-year-old male patient with small cell lung cancer (SCLC), whose marrow smears never showed a distinct morphology of metastasis. It was noteworthy that these SCLC cells accompanied the aberrant expression of CD45, leukocyte common antigen known as a specific marker for hematolymphoid neoplasms, which was not detected in the tumor of primary lesion. We describe this rare case to arouse an attention that tumors of non-hematolymphoid origin can exhibit exceptional CD45-positvity in metastatic sites.