Soft tissue sarcoma (STS) accounts for approximately 1% of all cancers diagnosed annually in the United States. Population-based data from Connecticut covering the years 1935-1989 have shown an increasing incidence of STS in both genders, with a greater increase among men than women. The recent increase in acquired immune deficiency syndrome-related Kaposi's sarcoma does not explain the upward trend in STS, dating back decades. Etiologic heterogeneity is suggested by epidemiologic variations that have been observed by subsite and cell type. Among the environmental factors associated with STS are external radiation therapy, Thorotrast, arsenical pesticides and medications, phenoxyherbicides, dioxin, vinyl chloride, immunosuppressive drugs, alkylating agents, androgen-anabolic steroids, human immunodeficiency virus, and human herpes virus type 8. In addition, STS occurs excessively among persons with certain heritable states including retinoblastoma, Li-Fraumeni syndrome, Gardner's syndrome, Werner's syndrome, nevoid basal cell carcinoma syndrome, neurofibromatosis type 1, and some immunodeficiency syndromes. These risk factors account for a minority of STS cases but provide leads for further epidemiologic and interdisciplinary studies into the genetic and environmental determinants of various forms of STS.