BACKGROUND Soft tissue sarcomas (STS) comprise a relatively rare group of malignant tumors of the connective tissues. It is estimated that approximately 6000 cases of STS will occur in the United States in 1993. Concerns have been raised in the clinical community that the incidence of STS may be increasing. METHODS Incidence rates of STS were obtained from the Surveillance, Epidemiology, and End Results program. Cases were restricted by histologic classification to malignant tumors determined a priori to be STS by a pathologist specializing in diseases of soft tissue. Age-, sex- and race-specific rates also were examined. RESULTS A total of 18,525 newly diagnosed cases of STS were identified for the period 1973 through 1987 in the United States. There was a marked increase in the age-adjusted incidence rate during the interval from 1981 to 1987, with rates increasing from 4.4 x 10(-6) in 1981 to 7.1 x 10(-6) in 1987. However, when patients with Kaposi sarcoma (KS) (International Classification of Diseases for Oncology [ICD-O] code 9140) were eliminated from this analysis, the rates remained relatively unchanged throughout the time period. Additional stratification by age, gender, and race revealed no important differences in this observed lack of a time trend. Gastrointestinal tract cases were examined separately, and no significant changes in incidence rates were observed. CONCLUSIONS It appears as though the increase in STS during this time period is accounted for primarily by KS, perhaps because of the acquired immune deficiency syndrome epidemic (since single men between the ages of 20 and 59 years accounted for almost 69% of all cases of KS). Additional studies that examine other populations or time periods after 1987 might be useful.