200 cases of thymoma operated on at the surgical centre of Marie-Lannelongue between 1955 and 1982 were reviewed and analysed statistically; their histology was compared and their evolutionary potential. Based on their degree of differentiation of the epithelial tumour component and on the proportion of non tumour associated lymphocytes, 4 histological types were defined: 1) Spindle-or-oval-cell thymomas where tumour cells reminiscent of normal involutive thymic cells were of small size with a tendency to form clusters, whorls or a glandular appearance; 2) Lymphocyte-rich thymomas where normal epithelial cells, few in number, reproduce the appearance of normal thymic cells; 3) Differentiated epithelial thymomas, comprised of large epithelial cells with abundant cytoplasm and clearly demarcated and often less numerous lymphocytes; 4) Undifferentiated epithelial thymomas, characterised by a poverty of lymphocytes and anaplastic epithelial cells with a raised nuclear cytoplasmic ratio, nuclear irregularity and mitosis. Statistically, there was a significant correlation between the degree of differentiation of the tumor and prognosis. Spindle-or-oval cell thymomas (type 1) and lymphocyte-rich thymomas (type 2) have an 80% survival at 5 years and a 75% survival at 10 years. Well differentiated epithelial thymomas of type 3 have a 75% survival at 5 years and a 50% survival at 10 years, finally undifferentiated epithelial thymomas (type 4); the survival rate was nil at 5 years.(ABSTRACT TRUNCATED AT 250 WORDS)