Two patients with chronic myelogenous leukemia (CML) who developed characteristic features of Behçet's disease (BD) during alpha-interferon (IFN-alpha) treatment and another patient who had a diagnosis of BD preceding CML are presented. In the first two patients, features of BD appeared 6 months after the initiation of IFN-alpha treatment: they included recurrent oral aphthae, genital ulceration, arthritis, folliculitis, and a positive skin pathergy test. The third patient, however, had a diagnosis of Behçet's disease 4 years before diagnosis of Philadelphia-positive CML. We prospectively examined the skin pathergy reaction in a group of patients with CML, multiple myeloma, and hairy cell leukemia both before and after IFN-alpha treatment and found two additional patients with CML who developed a positive skin pathergy test following IFN-alpha treatment.