The clinical course of non-severe aplastic anemia is variable, and risk factors related to disease progression are not well known. We reviewed clinical and laboratory data of the patients who were diagnosed with non-severe aplastic anemia from 1997 to 2007 at Seoul National University Hospital and analyzed the clinical course and outcomes in these patients. We defined non-severe aplastic anemia as hypocellular marrow with cytopenia in the peripheral blood, which does not meet the criteria for severe aplastic anemia (at least two of the following: ANC < 500/microl, platelet < 20,000/microl or reticulocyte < 20,000/microl). Among a total of 96 patients, 53 (55.2%) were male and the median age was 37.6 years old. As much as 41.7% (40) of the patients were initially asymptomatic. Sixty-two patients who were treated with oxymetholone, ATG/ALG, cyclosporin or other agents after initial diagnosis showed significantly lower levels of initial hemoglobin, red blood cell count and platelet count than those who did not receive any treatment. During the follow-up period, 18 patients progressed to severe aplastic anemia. Their median age was 29.9 years and the median progression time was 18 months. Initial white blood cell count and absolute neutrophil count in the evolution group tended to be lower than in the other group. The patients whose thrombocytopenia did not respond to treatment showed markedly higher frequency of progression to severe aplastic anemia. Treatment itself and responsiveness in reticulocyte and absolute neutrophil count were not correlated with their clinical courses. Sixteen patients showed overall improvement, whereas three patients developed secondary hematologic disease, acute myeloid leukemia, myelodysplastic syndrome and paroxysmal nocturnal hemoglobinuria. Non-severe aplastic anemia has a relatively indolent and mild clinical course. However, 18.8% of the study population progressed to severe disease. White blood cell and absolute neutrophil count at diagnosis and treatment responsiveness of thrombocytopenia were associated with disease progression. Careful monitoring and early management are needed for patients at risk.