Cytological findings of atypical adenomatous hyperplasia (AAH) in a 46-year-old man were compared to histological findings of biopsy specimens via fiberoptic bronchoscopy and videoassisted thoracoscopic surgery (VATS). The 12×10mm tumor, located at left S3, was diagnosed microscopically as AAH. Both the cytological specimen obtained by transbronchial needle aspiration and the histological specimens via transbronchial biopsy by bronchofiberscopy were evaluated. Cytological findings showed single atypical cells, discohesive clusters, and sheet-like arrangements. Round or oval nuclei were centrally located. and nuclear inclusion was often observed. Bi-and multinucleated atypical cells were seen, but not grooving or mitotic figurs, except rarely. These features may be important in distinguishing AAH from adenocarcinoma.