The relationship between 22 histologic variables and survival was investigated in 93 patients with acquired immunodeficiency syndrome (AIDS)-associated Kaposi's sarcoma (KS). All the patients were homosexual men in whom KS was the initial manifestation of AIDS. All patients were followed for at least 12 months or until death. Histologic specimens of the initial KS biopsy were reviewed in a blind manner by two of the authors and were evaluated for the presence of a number of histologic features. In a univariate analysis nodular lesions of KS (upsilon patch or plaque lesions), the absence of hemosiderin, the absence of irregular vascular spaces, and the presence of spindle cell nodules were all significantly associated with increased length of survival. Two variables previously shown to be related to survival (CD4:CD8 cell ratio, initial lesion on lower extremities) were included in a multivariate analysis (Cox model) in addition to the histologic variables. Complete data were available from 85 patients. In the multivariate analysis a higher helper to suppressor T-cell ratio, initial lesion on lower extremities, presence of spindle cell nodules, and nodular histology (upsilon patch or plaque histology) were all significantly associated with increased length of survival. These data suggest that in AIDS-associated KS, as in reticuloendothelial neoplasms, histologic features may be useful in identifying prognostically different subgroups of patients.