The records of 288 patients treated for small cell carcinomas of the lung are reviewed. Resectional surgery was performed in 150 cases (147 apparently curative and 3 palliative procedures), 73 patients were submitted to exploratory thoracotomy and 65 were deemed unsuitable for surgical treatment. Overall postoperative mortality after resection was 14,6% and 5-year survival rate 20,7%. Long term prognosis was significantly correlated to p-TNM (p less than 0,002) and host's immune response as evidenced by circulating peripheral lymphocytes/cumm (p less than 0,003), primary tumor lymphocytic infiltration (L.I.) (p less than 0.03), regional lymph-node reactivity (P.C.A., S.H. and C.A. development) (p less than 0,0001). Patients treated for Stage I disease and showing enhanced immune reactivity had a significantly better long term prognosis than Stage II, III and non reactive patients. Oat cell subtype had worse 5-year survival rates (10,7%) than intermediate cell subtype (25,4%), this difference however is devoid of statistical significance (p: not significant).