To define further the role of circulating immune complexes in acute schistosomiasis, serial determinations of [125I]C1q binding activity (C1qBA) in 15 patients with this disease were made before and after treatment with niridazole. Levels of C1qBA were elevated in 13 of these patients and in general rose early in the course of disease, reaching maximal levels by 10 weeks after exposure; the levels were correlated with disease severity (r = 0.70, P Schistosoma mansoni eggs in the stool (r = 0.59, P < 0.05). The rise and fall in levels of ClqBA was associated in most instances with the temporal course of other laboratory values reflecting disease activity. The close association of circulating immune complexes with measures of disease activity suggests that these complexes are important in the pathogenesis of acute schistosomiasis.