Using an MCLA-dependent chemiluminescence technique, we evaluated superoxide production by neutrophils isolated from 7 patients with Behçet's disease. After stimulation by phorbol myristate acetate, N-formyl-methionyl-leucyl-phenylalanine or opsonized zymosan, neutrophils from the patients produced significantly more superoxide than those from 20 controls. Pretreatment of control neutrophils with serum prominently enhanced superoxide production, and serum from Behçet's disease patients had a significantly greater effect than that from controls. These findings suggest that serum from patients with Behçet disease contains the priming factor(s) that can enhance enhanced superoxide production by neutrophils in response to stimulation.