Though not very common, solid tumor involvement of the bone marrow (BM) may have serious consequences. Recent studies have shown that mean platelet volume (MPV) is a good indicator for BM disease in the differential diagnosis of thrombocytopenia. We investigated the significance of MPV in the diagnosis of BM metastasis in patients with solid tumors. Patients with histologically-verified solid tumors for whom BM biopsy specimens were available (n = 121) and healthy controls (n = 62) were included in this retrospective study. A total of 183 individuals were analyzed. Of the patients, 61 had a diagnosis of BM metastasis (Group A), 60 did not have BM metastasis (Group B). Group B and C (healthy controls) constituted the control group without BM metastasis (n = 122). The mean MPV was 7.0 ± 0.8 fl in patients with BM metastasis and 8.4 fl in the control group (P < 0.001). A cut-off point of <7.4 fl was found to have significant predictive value according to receiver-operating characteristics curve analysis. This cut-off point had 85% positive predictive value and 90% negative predictive value in the diagnosis of BM metastasis (odds ratio: 53; 95% confidence interval: 20–135), and a sensitivity of 82.7% and specificity of 89.6%. MPV can be used as a reliable marker to guide the clinician as to the likely presence or absence of BM metastasis in patients with solid tumors.