Fifty patients with nonresectable lung cancer were treated with a combination of cyclophosphamide, vincristine, methotrexate and procarbazine (COMP). Twentyfour patients (50%) of 48 evaluable patients responded to the therapy. Immune function, including total lymphocyte counts, T and B lymphocyte counts, in vitro lymphocyte blastogenesis by PHA and PPD and PHA skin reaction, were evealuated serially in reference to intensive cancer chemotherapy. The results obtained were as follows: 1) Although total lymphocyte counts, T and B lymphocytes counts, and in vitro blastgenic activity by PHA were significantly decreased immediately after COMP therapy, these parameters recovered to pretreatment level approximately within 3 to 4 weeks interval to next COMP therapy, there were no significant changes in PPD and PHA skin reaction following COMP therapy. 2) There was no relationship between pretreatment immune function and response to COMP therapy. 3) Recovery from impaired immune function was noted occasionally among the responders to COMP therapy. 4) A close relationship was noted between pretherapy response to PHA skin reaction, as well as pretherapy performance status, and survival of stage Ⅲ lung cancer patients treated with COMP therapy.