Primary lung cancers (479) were typed according to the WHO histological classification. The character of the material and the methods of investigation are described. All patients were subjected to mediastinoscopy and 313 patients were operated on. Nearly half of the tumors (48%) were epidermoid carcinomas. Small cell anaplastic carcinoma occurred in 25% and .apprx. 2/3 of these were of oat cell type. Adenocarcinoma was found in 22% and the acinar type predominated. Bronchioloalveolar carcinoma occurred in 1% and large cell carcinoma in 3%. Typing of biopsy specimens was made in 289 cases in which a positive biopsy was obtained during the pretreatment period. The result of the biopsy typing was checked against that of the final one. In the total group the preoperative histological diagnosis tallied with the final one in 88%. In patients subjected to surgery the pretreatment diagnosis of epidermoid carcinoma was correct in 86%, that of small cell anaplastic carcinoma in 92% and that of adenocarcinoma in 100%. The consistency was also high in the category of patients not subjected to surgery. Despite their critical attitude toward the delimitation of epidermoid carcinoma in the WHO classification, the present workers find it to be a reliable guide to routine typing of lung cancer.