Abstract: Objective To explore the clinical and pathological characteristics of involvement of bone marrow metastasis with an unknown primary and analyze the significance of laboratory examinations. Methods Clinical data about 33 patients admitted to our hospital from January 2009 to January 2014 who were diagnosed to have cancer of unknown primary were retrospectively analyzed. The non-haematological disorders were analyzed, including clinical features, bone marrow and peripheral blood smear cytological examinations, hematological and biochemical parameters, and tumor markers. Results Bone pain (63.4%) and anemia (54.6%) were the most frequent clinical findings followed by fever with unknown origin, cough, fatigue and weight loss. Laboratory findings showed anemia and thrombocytopenia occurred in patients, and leukoerythroblasis was observed on the peripheral blood smears. In addition, ALT, AST, LDH and ALP increased. Fifty-five percent of patients were found to have hypoprotienemia, and CEA was elevated in 76 percent of patients. The non-haematological tumors cells were typical, which were presented as variable number, size, uneven distribution and morphological abnormalities. Conclusion Bone marrow examination is a useful and cost-effective laboratory tool in asserting the diagnosis of MCBM. The hematological parameters are of great significance to predict the likelihood bone marrow involvement.