Parallel cytological and histological studies were performed for 87 patients with pulmonary adenocarcinoma. Sputum, impression smears of the bronchial mucosa, aspirates from the bronchi, punctates of the tumor obtained transbronchially, pieces of tumor obtained at bronchoscopy, and operation material were examined. Considerable difficulties wre found in cytological diagnosis of adenocarcinomas due to degenerative changes in tumor cells, the presence of cells of the metaplastic columnar epithelium and marked anaplasia of tumor cells in adenocarcinoma with low differentiation. The frequency of diagnostic errors was 20.0--50.6% depending on the histological form of adenocarcinoma and the accompanying processes in bronchi. The cytological criteria improving the differential diagnosis of adenocarcinoma are presented. The validity of cytological diagnosis of adenocarcinoma does not depend on the nature of the material examined.