In order to estimate the possibility of a cytologic differential diagnosis of bronchiolo-alveolar carcinoma and bronchogenic adenocarcinoma, 121 histologically proven cases of these two types of lung cancer were studied. Certain features of the tumor cells were used as differential diagnostic parameters. By means of these features it was possible to correctly type 90% of the bronchiolo-alveolar carcinomas and 72% of the bronchogenic adenocarcinomas. The most striking characteristics of bronchiolo-alveolar carcinoma was the uniformity of cells, the occurrence of such cells in tightly packed clusters, the absence of prominent nucleoli and also the scarcity of single tumor cells. Thus, it seems possible to make a correct cytologic differential diagnosis between these two types of lung tumors with high accuracy.