A series of 24 lung cancer cases was studied: 12 epidermoid carcinomas, 9 adenocarcinomas, 2 giant-cell carcinomas and 1 carcinoid. The patients were staged on the basis of the TNM classification system as 9 stage I, 5 stage II, 9 stage III and 1 stage IV. Using fresh tumour cell samples 2 cell cultures were prepared for each patient: one to identify the percentage of S phase cells (Labelling Index) using the tritiated thymidine method and one for cytogenetic analysis. A gentic map was obtained in 6 cases and revealed no specific numerical or structural alterations. The Labelling Index (L.I.) was calculated for all patients and compared with all TNM parameters. This revealed a certain connection between L.I. and parameters T, SN and G but no link with parameters.