Objective : To identify key cytologic features for diagnosis of adenocarcinoma and morphologic differentiation from reactive/reparative respiratory epithelium. Study Design : The cytomorphologic features of 145 pulmonary cytology specimens (sputum and bronchial washing), which included 117 histologically proven adenocarcinomas and 28 non-neoplastic lesions, cytologically diagnosed as atypia and suspicious for malignancy (adenocarcinoma) were reviewed retrospectively. We analyzed 11 morphologic criteria in pulmonary cytologic specimens. Results: Over 110 of 117 cases of adenocarcinomas revealed nuclear membrane irregularities, non-cohesive cells, single atypical cells, moderate to markedly enlarged nuclei and an increased nuclear/cytoplasmic (N/C) ratio. Chromatin clearing, chromatin heterogeneity and hyperchromasia were seen in 102, 99 and 97 cases, respectively. All 28 cases involving non-neoplastic lesions revealed hyperplastic reactive pneumocytes, hyperplastic reactive bronchial epithelium, or degenerating macrophages. The non-neoplastic lesions revealed a small number of atypical cell clusters and paucity or absence of atypical single cells. Conclusion : The most important morphologic features for diagnosis of adenocarcinoma are nuclear membrane irregularities, non-cohesive cells, single atypical cell, moderate to markedly enlarged nuclei and increased N/C ratio. Chromatin clearing, chromatin heterogeneity and hyperchromasia were also helpful features, while pleomorphism and prominent nucleoli were less valuable.