Infiltrating (intramuscular) lipomas and angiolipomas are benign mesenchymal tumors that usually appear as a deep, nontender mass within soft tissue, particularly in the extremities. The average tumor size in six cases studied was 11.2 cm (range, 2.0 to 22.0 cm). On gross examination, these tumors are circumscribed but unencapsulated, with infiltration of adjacent skeletal muscle. The correct preoperative diagnosis is seldom made, and the characteristic infiltrating pattern seen microscopically can lead to a mistaken diagnosis of sarcoma. Soft-tissue roentgenograms can be helpful in diagnosis and localization. The recommended mode of therapy is complete local excision with tumor-free soft-tissue margins. None of the six patients described here have experienced recurrence of tumor an average of two years after surgical resection. Prolonged follow-up is recommended, however, since inadequate resection can result in late tumor recurrence.