In order to determine whether abnormalities of immunoregulatory T-cells occur in patients with lung cancer, we characterized peripheral T-lymphocytes in 26 patients with untreated lung cancer. The results in patients with primary squamous cancer (SC) (n = 10), primary adenocarcinoma (AC) (n = 7), and secondary lung metastases (M) (n = 9) were compared with each other and to subjects without cancer (n = 48), including nonsmokers (n = 29) and smokers (n = 19). We found that OKT3+ (mature, peripheral (T-lymphocytes, including both OkT4+ (inducer/helper) and OKT8+ (cytotoxic/suppressor) lymphocytes, were increased in light-to-moderate smokers, but that OKT4+ cells were decreased n heavy smokers (p less than 0.05). The ratio of OKT4+ to OKT8+ (4/8) lymphocytes, reflecting the balance of immunoregulatory cells, was normal in light-to-moderate smokers, but was decreased in heavy smokers (p less than 0.05). The profile of circulating T-cells in patients with SC was similar to the smokers. In contrast, in patients with AC, we found a decreased percentage of OKT8+ cells (p less than 0.05). The 4/8 ratio was elevated in patients with AC (p less than 0.05). In patients with M, there was a decreased percentage of OKT3+ cells reflected in both OKT4+ and OKT8+ subsets. The 4/8 ratio in patients with M was low. Thus, a number of abnormalities in circulating T-cells was found both in smokers and in patients with lung cancer. These results suggest that immunoregulatory abnormalities contribute to the pathogenesis of lung cancer.