Natural Killer cell activity and lymphocyte subset in peripheral blood were studied in 16 patients with Crohn's disease. The effect of interferon administration was evaluated in 4 patients with Crohn's disease. In the patients with active Crohn's disease, natural killer cell activity was significantly lower in level compared to that in normal controls. Those without increase of natural killer cell activity in non-active stage, had trend of early relapsing. Peripheral lymphocyte subpopulations of OKT4, OKT8, OKIal, OKT4/8 and Leu7 by flow-cytometry showed no significant differences between the patients and the normal controls. Clinical remission was maintained in 2 out of 4 patients for the period from 7 to 21 months after the interferon administration. Benefit of interferon administration was suggested in patients with Crohn's disease.