Gynecomastia is a common benign male breast disease, which may exhibit mild cellular atypia in cytology specimens. However, marked cytologic atypia can be seen in gynecomastia superimposed by chemotherapy. The case described in this report demonstrated severe cytologic atypia of gynecomastia mimicking carcinoma in a patient treated with chemotherapy for acute leukemia. A distinct cytologic feature helpful in avoiding the diagnostic error is described, namely, atypical cells admixed with bland ductal cells and appearing at a different plane. The importance of applying strict diagnostic criteria in breast cytology and clinical correlation is also emphasized.