This clinicopathologic study concerns 42 cases of synovial sarcoma (13 biphasic and 29 monophasic), including electron-microscopic examination of five cases, among 753 cases of soft tissue sarcomas. The age of the patients ranged from 9 to 70 years, with a median of 35 years. Tumors occurred most commonly on the extremities with 31 on the lower extremities. Histologically the monophasic type on this occasion included tumors with focal or minimal biphasic differentiation (9 cases) in addition to totally monophasic tumor (20 cases). A comparative light microscopy revealed otherwise inappreciable differences in histologic characteristics between the monophasic synovial sarcoma and certain other spindle cell sarcomas. Ultrastructurally, the cells composing the spindle-cell area of the synovial sarcoma contained basically the same elements as did the cells forming epithelioid or gland-like structures, and as the cells in the areas of gradual transitions of the two. One exception was that the cells lining the gland-like lumina had microvillar projections. Characteristic secretory-like granules, similar to those seen in the synovial type B cell of the normal human synovium, were evident in all five cases studied electron microscopically. In contrast to the findings of others, patients with monophasic tumors had a better prognosis, 58% surviving 5 years compared to 36% for those with biphasic tumors.