One hundred five patients with advanced non-small cell lung cancer were randomized to receive thymosin fraction V immunotherapy during remission induction chemotherapy with vindesine, doxorubicin, and cisplatin (VAP). Fifty-four patients received VAP alone. Fifty-one patients received VAP + thymosin. Both groups were comparable; most patients were male, with a good performance status and with the diagnosis of adenocarcinoma. Among 99 evaluable patients, response was seen in 24 (2 CRs, 22 PRs) of 53 (45%) patients treated with VAP and 10 (all PRs) of 46 (22%) patients treated with VAP + thymosin (p = 0.03). VAP-treated patients responded better than those treated with VAP + thymosin in each tumor category: adenocarcinoma, 50% of 36 patients versus 22% of 27 patients; squamous cell carcinoma, 29% of 14 patients versus 21% of 13 patients; undifferentiated carcinoma, 67% of three patients versus 17% of six patients. Median survival duration was 34 weeks versus 25 weeks in favor of the VAP-treated group (p = 0.14). Thymosin treatment resulted in decreased graft-vs.-host reaction (p = 0.01) and increased suppressor effect on normal mitogen response to Con-A (p = 0.17). The activity of VAP chemotherapy is comparable with the most effective multidrug regimens of the present time in patients with advanced non-small cell tumors. The addition of thymosin immunotherapy appeared to have a negative effect on the activity of VAP.