The pathogenesis of thymoma is unclear. In this study retrovirus-like particles in human thymomas were detected by electron microscopy. Forty-two thymomas; 25 without complications and 17 associated with autoimmune disorders such as myasthenia gravis (13), systemic lupus erythematosus (1), polymyositis (1), Sjögren's syndrome (1), and pure red cell anemia (1), were examined. Thymic tissues from 9 infants suffering from congenital heart diseases and 7 hyperplastic thymuses obtained from myasthenic patients served as controls. The retrovirus-like particles were observed in 37.0% of thymomas without complications; 50.0% of thymomas associated with myasthenia gravis and other autoimmune disorders; 62.5% of thymuses associated with myasthenia gravis; and 33.3% of thymuses from infants with heart disease. The envelopes, including the central cores of the retrovirus-like particles, had diameters ranging from 70 to 460 nm, depending on the source of the specimen. The retrovirus-like particles were located in the cytoplasm, vacuoles, vesicles and lumens of the endoplasmic reticula of epithelial and/or plasma cells. Some retrovirus-like particles were seen budding from plasma membranes. These observations suggest that the retrovirus-like particles in thymomas might be an activated form of retrovirus originating in normal thymic tissue.