Natural killer cell (NK) activity and antibody-dependent cellular cytotoxicity (ADCC) exerted by peripheral blood mononuclear cells (PMNC) were investigated in 53 patients with Crohn’s disease (CD). NK activity and ADCC were found to be significantly reduced in patients with CD (p < 0.0005) as compared to healthy controls. Both effector cell functions increased after in vitro treatment of PMNC with n-interferon, but did not reach the levels found in controls (p < 0.0005). Neither NK activity nor ADCC was significantly influenced by therapy with corticosteroids. Moreover, the reduced serum zinc levels in patients with CD, which have been shown to be associated with impaired immune function, did not influence the lytic effector cell mechanism assayed either. Finally, no association could be found between NK cell activity or ADCC and CD activity index, the extent of the disease and several laboratory parameters of inflammation. We conclude that patients with CD have a reduced lytic effector cell function which remains uninfluenced by corticosteroid treatment and seems to be present independently of disease activit