To evaluate the role of fine needle aspiration cytology (FNAC) in identifying in situ breast carcinoma, we reviewed 19 FNACs of histologically confirmed pure or predominantly ductal carcinoma in situ (DCIS). The cytologic diagnosis was positive for malignancy in 53%, suspicious in 31% and nondiagnostic in 16%. An intraductal lesion was suggested prospectively in 21%. Retrospective review showed three distinctive cytologic criteria in cases with DCIS: (1) cohesive groups of atypical ductal cells associated with scattered, individual malignant cells or a necrotic background; (2) hyperplastic ductal cells with associated malignant cells or necrosis; and (3) true tissue fragments composed of cohesive epithelial cells with a cribriform pattern. One or more findings were present in 81% of the malignant or suspicious FNACs; 19% could not be distinguished cytologically from invasive carcinoma. We studied a control group of 30 invasive ductal carcinomas; one or more criteria were found in 35% of cases with no or a minor DCIS component but in 73% of those with an extensive DCIS component. We conclude that these three criteria deserve further study as an aid in suggesting DCIS on FNAC.