Thirty-one patients with acute schistosomiasis were evaluated clinically and immunologically. Cytokine levels were determined in peripheral blood mononuclear cell (PBMC) supernatants. Levels of total and antigen-specific IgE, tumor necrosis factor (TNF)-alpha, and immune complexes were measured in serum samples. Clinical findings included general symptoms, liver damage, pulmonary involvement, and pericarditis. All patients had eosinophilia. Immune complexes were detected in 55% of the patients (mean+/-SD, 7.8+/-7.6 microg Eq/mL) and were associated with cough, dyspnea, and abnormal chest radiographic findings. Levels (mean +/- SD) of TNF-alpha (1349.3+/-767.6 pg/mL), interleukin (IL)-1 (2683+/-1270 pg/mL), and IL-6 (382 +/- 52.3 pg/mL) were elevated in PBMC. Serum TNF-alpha levels were elevated in 87% of the patients and were associated with abdominal pain. Higher interferon-gamma levels were detected in PBMC of patients with acute disease than in those of patients with chronic schistosomiasis; IL-5 levels were higher in those with chronic disease. Low IL-5 levels were associated with weight loss. Proinflammatory cytokines and immune complexes with low Th2 responses might explain the immunopathogenesis of acute schistosomiasis.