The most important prognostic determinants of the thymomas are the gross findings at operation (equal to the presence or absence of gross invasion of adjacent tissue) and the presence or absence of the thymoma-associated systemic disease, particulary myasthenia gravis. The histologic type of thymoma had no proof value in predicting prognosis with the exception of the so-called atypical thymomas. Thirty-four of 57 patients with thymomas were females and 23 males. The tumors in 40 cases were non-invasive thymomas, and in 17 cases the tumour were invasive of adjacent tissue. 35.1 percent of patients were asymptomatic, the tumours being discovered on roentgenograms done on a routine basis or for an unrelated porpose. 40.3 percent of patients have had a thymoma-associated systemic disease. The most common presenting symptoms were related to myasthenia gravis (26.3%). The 5-year survival rate was 90 percent for non-invasive thymomas without myastenia gravis and 50 percent for invasive thymomas. The 5-year survival rate for patients with myasthenia gravis and encapsulated (non-invasive) thymomas was approximately 60 percent, whereas that for invasive thymomas with myasthenia gravis was 40 percent. The primary form of therapy for all thymomas is the surgical excision. In cases with invasive and/or metastasizing thymomas, postoperative radiation and adjuvanted chemotherapy is indicated, but in most series, the longterm results of this form of therapy are discouraging.