Twenty-three patients underwent reoperation for thymoma at intervals of 2 months to 17 years 10 months after the initial operation. There were no operative or hospital deaths. Myasthenia gravis occurred in 12 patients, but in only 2 was it a determinant for reoperation. The longest survival after reoperation is 12 years 9 months, and that patient is free from tumor. Four distinct surgical groups emerged, and their recognition provides an improved method of reporting and suggests a strategy for better overall management. Group 1 (n = 5) had completion of thymectomy (reoperation) after thymomectomy alone or after incomplete thymectomy. The interval was 2 months to 17 years 10 months. All 5 had myasthenia gravis. At reoperation, thymomas were found in 3 and a hyperplastic thymus in 2. Four are alive and tumor free 2 years to 8 years 2 months after reoperation. One died tumor free after 5 years. Group 2 (n = 8) had reoperation for recurrent thymoma after standard (presumably complete) resection. The interval was 2 years to 13 1/2 years. Four had myasthenia gravis. Four are alive 8 months to 5 years 8 months after reoperation, 3 without detectable tumor. Four died 3 years 3 months to 8 years 4 months after reoperation, 3 free from tumor. Group 3 (n = 8) underwent reoperation for initially unresectable thymoma after adjuvant treatment with chemotherapy, radiotherapy, or both. The interval was 3 months to 4 years 8 months. Three had myasthenia gravis. Six are alive 4 months to 4 years after reoperation, only 1 with tumor.(ABSTRACT TRUNCATED AT 250 WORDS)