Objective. Hyperfunction of neutrophils is a characteristic finding in Behcet's disease (BD). Microbial agents have been proposed as causative agents in the disease flares. Fc gamma receptor 1 (CD64) is not normally expressed by neutrophils of healthy individuals, but is upregulated by these cells in response to microbial wall components and proinflammatory cytokines. The degree of polymorphonuclear leukocyte (PMN) CD64 expression is different in autoimmune diseases and systemic infectious diseases. We investigated PMN CD64 expression in patients with BD.