Summary.-The post-operative survival in 554 lung carcinomata, classified according to the histological type, was calculated by the actuarial method. On the whole, squamous cell carcinoma was the most favourable and anaplastic small cell carcinoma the least favourable lesion. However, in tumours smaller than 4 cm, confined to the lung and with negative lymph nodes (stage I), small cell carcinoma had the highest percentage of 5 year survivors, followed by large cell carcinoma, squamous cell carcinoma and adenocarcinoma. When tumours had attained a larger size and/or spread tQ neighbouring structures and regional lymph nodes (stage II and III), the histological type was a much more determining factor in survival, squamous cell carcinoma being a significantly more favourable lesion. On the other hand, no difference in survival in relation to the histological type was found when distant metastases were probably present (stage IV). It was concluded that in assessing the role of histopathology in the prognosis of lung cancer, the mutual relationship to other pathological factors must be taken into account.