BACKGROUND: Hypocellularity of bone marrow (BM), not associated with significant dyshematopoiesis, is often found in patients with isolated thrombocytopenia, but its clinical implications have not been studied. We prospectively studied the clinical features and natural history of these patients. METHODS: Adults with isolated thrombocytopenia (platelet counts <100x10(9)/L) in the absence of dyshematopoiesis, cytogenetic abnormalities, or megakaryocytic hyperplasia and who had BM hypocellularity (below 30% in patients aged less than 60 years; below 20% in patients aged 60 years or more) were enrolled at Chungnam National University Hospital between January 2002 and December 2006. They were monitored regularly for changes in platelet counts or development of additional cytopenia. RESULTS: Twenty patients (17 men and 3 women) were enrolled in the study. The median age was 29 years (range, 18-70 years). At initial presentation, the platelet counts ranged from 12x10(9)/L to 99x10(9)/L (median, 63x10(9)/L) and were >50x10(9)/L in 16 patients (80%). BM cellularity ranged from 5% to 25% (median, 15%) and was < or =10% in 6 patients (30%). During the median 48-month follow-up (range, 12-90 months), platelet counts of 3 of the 20 patients recovered to normal levels (>150x10(9)/L) after 12, 56 and 66 months. Three patients developed pancytopenia after 11, 70 and 90 months. Two patients were consistent with moderate aplastic anemia, and 1 was confirmed as having refractory cytopenia with multilineage dysplasia. In the remainder of the patients, platelet counts remained unchanged. CONCLUSION: Isolated thrombocytopenia accompanied by hypocellular marrow encompasses a group of heterogeneous conditions.