A 41-year-old man with Sweet's syndrome (SS) had symptoms similar to Behçet's disease (BD). To study the relationship of the two diseases, human leukocyte antigen (HLA) typing was performed on 28 patients with SS and 49 patients with BD. Of the 28 patients with SS, seven had BD symptoms. The frequencies of both HLA-B51 and -DQw3 were significantly higher in patients with BD. However, the frequencies of the two HLA antigens in the 28 patients with SS and the 21 patients with SS without BD symptoms were not significantly different from the controls. The frequency of HLA-Bw54 was significantly increased in both groups of patients with SS. Taken together, these data indicate that SS is a genetically distinct disease entity from BD, although their symptoms are similar and the incidence of SS among patients with BD is high in Japan.