This investigation was intended to determine whether the natural killer (NK) activity of peripheral blood lymphocytes (PBL) correlated with the histopathological factor, which is thought to be a result of a balance between tumor aggression and host resistance. The NK activity of PBL from 60 patients with lung cancer was measured by the lysis of 51Cr-labelled K562 target cells. The activity was significantly decreased with advancing stages of the disease, and inversely correlated with increased immunosuppressive substance levels of the serum. Histopathological factors, such as low grade pleural invasion of the tumor and abundant lymphoid cell infiltration around the tumor, were significantly associated with the high NK activity of PBL. These results show that a decrease in NK activity may play a role in identifying those individuals with a greater risk of cancer development.