Serum RNase levels were determined in 54 patients with lung carcinoma, 74 long-term cigarette smokers and 172 nonsmokers. The mean serum RNase level was significantly higher in patients with lung carcinoma and long-term smokers compared with healthy nonsmokers (P < .001). The serum RNase activity level was not related to chronological age, sex, or race of the smoker or nonsmoker population. Forty (75%) of 53 patients with lung cancer and 49 (66%) of 74 smokers had elevated serum RNase levels compared with 13 (7%) of 179 nonsmoker healthy controls (P < .001). The highest incidence of elevation was noted in patients with epidermoid carcinoma (95%).