We evaluated the changes of peripheral leukocyte counts, lymphocyte counts, and percentage of lymphocytes and T cell subsets (OKT4(%), OKT8(%) and OKT4/OKT8) in 49 untreated lung cancer patients of whom 23 were followed up after therapy (8 pulmonary resection, 15 chemotherapy). Total WBC counts were significantly increased in stage I or II compared to controls and were decreased after operation. Total counts and percentage of lymphocytes were decreased (esp. stage III NSCLC, SCLC esp. ED, squamous) and were persistently decreased after treatment in nonresponders. OKT8(%) was decreased (esp. SCLC, squamous, adenocarcinoma, all stages of NSCLC, and ED of SCLC) and was persistently decreased in stage III after treatment. The OKT4/OKT8 ratio was increased (esp. adenocarcinoma and stage III NSCLC) and returned to the normal range after treatment. In nonresponders, total lymphocyte counts were decreased before treatment compared to responders and persistently decreased before and after treatment compared to controls, and OKT8(%) was persistently decreased after treatment compared to controls. In conclusion, immunoabnormalities in patients with lung cancer might be improved following treatment.