We studied 473 unselected patients with thrombocytopenia. The mean platelet volume (MPV) was 8.1 fl in patients with marrow disease and 9.8 fl in patients without marrow disease (P < 0.001). A total of 5% of patients with an MPV >or=10.5 fl have marrow disease (odds ratio 0.05, 95% CI 0.02-0.13). Conversely over three quarters of patients with an MPV of <8.0 fl have marrow disease (odds ratio 8.1, 95% CI 5.0-13.0). Therefore the MPV can strongly guide the clinician as to the likely presence or absence of bone marrow disease in thrombocytopenic patients.