Serum leucine sminopeptidase (LAP) activity and its isozyme fraction were assayed in 73 primary lung cancer patients with the passage of time before the operation to 3 years after the operation. The LAP activity showed a high value in 25% of the patients before the operation, but did not exhibit a significant change after the operation. By contrast, the isozyme Y fraction showed a high value in 71% of the patients before thee operation and normalization was observed in half of them 1 month after the operation. There was no significant difference in the Y fraction with the size of tumor before the operation, but a significant fall wes observed after curative resection. About half of the patients in whom the Y fraction showed a high value before the operation died within 1 year after the operation. Furthemore, the subsequent prognosis was unfavorable evan in patients in whom the value was normalized after the operation, and a high Y fraction value before the operation seemed to serve as a parameter of unfavorable prognosis.