The micropapillary pattern of lung adenocarcinoma was discussed in the 2004 World Health Organization classification and is now proposed as a distinct pattern in the new International Multidisciplinary Classification of Lung Adenocarcinoma Guidelines. The micropapillary pattern is histologically characterized by papillary tufts lacking a central fibrovascular core and is associated with an unfavorable prognosis. Herein, we report the cytological features of lung adenocarcinoma with the micropapillary pattern in a sputum specimen. A 75-year-old woman presented with a productive cough, blood-tinged sputum, and some symptoms of upper respiratory tract infection. The initial impressions from her chest radiograph and computed tomography scan indicated pneumonia. However, the initial sputum cytology sample showed a few clusters of cells with abnormal three-dimensional structure, interpreted as adenocarcinoma. These cells were small and had minimal cytologic atypia, demonstrating a potential diagnostic pitfall. The following biopsy confirmed lung adenocarcinoma with the micropapillary pattern. Here, we describe this case and discuss the differential diagnosis of pulmonary entities exhibiting similar morphologies. Diagn. Cytopathol. 2014;42:902–905. © 2014 Wiley Periodicals, Inc.