A spontaneous multifocal subcutaneous fibromatosis is described in 6 pig-tailed macaques (Macaca nemestrina) with simian acquired immune deficiency syndrome (simian AIDS). The lesions consisted of a proliferation of vascular fibrous tissue that was infiltrated by lymphocytes and plasma cells. One animal also had retroperitoneal fibromatosis, which has also been found in this colony of pig-tailed macaques. Progressive weight loss, diarrhea, lymphadenopathy, and neutropenia were seen. Peripheral lymph nodes were hyperplastic, and there was splenomegaly. Aggregates of lymphocytes were present in the bone marrow, kidneys, liver, and lungs. Type D retrovirus particles were found in three nodules by electron microscopy; intracytoplasmic type A and budding particles were identified in fibroblasts. In a setting of acquired immunodeficiency, these subcutaneous tumors in pig-tailed macaques present a striking analogy to Kaposi's sarcoma in human AIDS.