This study evaluated the cytotoxic activity of natural killer cells in the active and inactive stages of Behçet's disease (BD) and attempted to develop a new explanation for its immunopathogenesis. Blood samples were taken from 16 BD patients and compared with 11 healthy individuals. The lymphocyte fraction was separated and diluted in RPMI-1640. Candida as a target cell (T) was mixed with lymphocytes (E) (effector cells) in ratios of T:E 1/5 and T:E 1/25. After the numbers of colonies were counted with controls, the anticandidal index (natural cytotoxicity) was calculated. Natural cytotoxicity relatively decreased in the active stage and increased in the inactive stage of BD. Although the difference between the mean value of natural cytotoxicity in the active stage and in the inactive stage was significant, the difference between the averages of active stage and the control group was insignificant. However, the difference between inactive stage and the control group was remarkable. The increase of the natural cytotoxic activity in the inactive period of the disease may play a role together with other immune mechanisms in the aetiopathogenesis of BD.