Twenty one patients of malignant thymoma treated with curative aim at the Department of Therapeutic Radiology of Seoul National University Hospital from 1979 to 1987 were analysed retrospectively. The 3 year overall and relapse free survival rate was 80.5% and 78.6%, respectively. Myasthenia gravis (Mg) was seen in 43.5% at presentation and disappeared in 40% (4/10) after radiotherapy with or without operation. The 3 year cumulative survival rate with and without Mg was 90% and 78.8%, respectively. We could consider that Mg was no longer abverse prognostic factor. The complete response rate after partial resection was 100% (3/3), and that after biopsy was 20% (3/15). The overall local control rate including complete and partial response rate (33% vs 56%) was 89% and the 3 year actuarial survival rate by the response rate was 88.9% and 81.7%, respectively. There was no statistically significant survival difference between two groups. The crude rate of relapse at 3 years was 23.8% (5/21), and 80% (4/5) were locoregional failures. All failures were observed in biopsy only group, while no failure was observed in resected group. The major pattern of the treatment failure was the locoregional failure and the distant metastases was rarely observed.