Twenty-five patients with malignant invasive stage III thymomas who underwent biopsy for tissue diagnosis were treated with primary radiotherapy (mean dose 46.36 Gy, range 32.4-58 Gy). These patients were followed up for a period of 10 years and survival/failure analysis was performed. Five prognostic variables were compared using the log rank test. There was no difference in survival between ages less than 50 and more than 50 years, presence or absence of myasthenia gravis, sex, histology and race. The mean follow-up was 39 months (range 1-86 months). The 5-year disease-free survival was 81% overall survival 72% and local failure rate 13%. Most local failures occurred in the first 3 months. Six patients died after a course of radiotherapy (2 intrathoracic relapse, 1 disseminated disease, 1 local failure and distant metastasis, 2 causes not related to disease). Hilar fibrosis was seen in 4 patients who are asymptomatic. No other complications were recorded. Radical external beam radiotherapy alone can give good results in malignant stage III invasive thymomas.