Natural killer cell (NK) activity of peripheral blood lymphocytes against K 562 cells was investigated in lichen planus (LP). 38 LP patients with cutaneous or oral mucosal involvement and 20 healthy controls participated in the study. A statistically significant decrease in the NK response in LP patients with extensive erosive oral mucosal involvement (p < 0.02) and in generalized acute eruptive LP (p < 0.01) was noted compared to those with nonerosive LP of the oral mucosa or healthy controls. Interleukin 2 failed to restore completely the reduced NK activities in our patients with LP.