It has been recently reported that the soluble interleukin-2 receptor (IL-2R) levels in the sera of cancer patients were higher than those of normal controls. The present study was conducted in order to clarify the clinical significance of serum soluble IL-2R in patients with lung cancer. Using commercially available EIA kits, we measured the serum levels of soluble IL-2R in 102 lung cancer patients and 18 normal controls. The serum level of IL-2R was higher than 100 pM (mean +3 S.D. in the normal controls) in 14 of 58 patients with adenocarcinoma and in 13 of 32 patients with squamous cell carcinoma. In both adenocarcinoma and squamous cell carcinoma, the mean level of soluble IL-2R was higher in advanced stages (Stages IIIA, IIIB and IV) than in early stages (Stages I and II). In contrast, no patients with small cell carcinoma exhibited a serum level of soluble IL-2R higher than 100 pM, whereas almost all of those patients were in advanced-stage diseases. These results first demonstrated that the serum level of soluble IL-2R increased in association with both the disease stage and the histological type in lung cancer.