OBJECTIVES--To assess the effect of surgical cleaning of the skin on the pathergy phenomenon in Behçet's syndrome. METHODS--The pathergy phenomenon was assessed in 183 patients with Behçet's syndrome in a single blind study at 48 hours after a needle prick of the forearm skin, cleaned in the conventional way with alcohol, with 10% povidine iodine, with 100% chlorhexidine, and with a 4% aqueous solution of chlorhexidine. RESULTS--The surgical cleaning of the forearm with povidone iodine before the application of the needle prick reduced the prevalence of the pathergy reaction to 27% from 48% in the conventionally (by alcohol swab) cleaned forearm. Cleaning the forearm with 100% chlorhexidine reduced the prevalence of the pathergy to 36% from 59% in the conventionally cleaned forearm. No significant changes were observed with a 4% aqueous solution of chlorhexidine. CONCLUSIONS--Surgical cleaning of the skin with disinfectants of various concentrations reduces the prevalence of a positive pathergy test in Behçet's syndrome. This implies that more than the disruption of the structural integrity of the dermis and epidermis is involved in the production of the pathergy phenomenon in Behçet's syndrome.