The prognostic implications of cigarette smoking were investigated in 112 patients with small cell lung cancer. Twenty had stopped smoking permanently before diagnosis (NS-Prior), 35 had stopped at diagnosis (NS-Dx), and 57 patients continued smoking (S). Therapies included chemotherapy alone or with radiation therapy, with or without thymosin fraction V. The survival difference among the three groups was statistically significant. The NS-Prior patients had the best survival, followed by NS-Dx patients and finally S patients. No S patient has survived, disease free, more than 96 weeks, while three NS-Prior and three NS-Dx patients are disease free 103 to 220 weeks after start of treatment. Thymosin, 60 mg/sq m, yielded survival benefits for the S group only. Continuation of smoking during the treatment of small cell lung cancer was associated with a poor prognosis, while discontinuation of smoking, even at diagnosis, may have beneficial effects on survival.