BACKGROUND Tumors of the pancreas associated with extracellular mucin production include mucin-producing ductal adenocarcinoma, mucinous cystic neoplasm (MCN), and intraductal papillary mucinous tumor (IPMT). Fine-needle aspiration (FNA) is used as an adjunct to radiologic analysis for the preoperative categorization of these tumors. The current study was designed to identify distinctive cytomorphologic features that would be useful for the categorization of mucinous tumors of the pancreas. METHODS The authors evaluated Papanicolaou smears and Diff-Quik-stained smears of specimens obtained by computed tomography-guided and endoscopic ultrasound-guided FNA of the pancreas in 51 cases of mucinous tumors. In the 19 cases in which the patients underwent surgical excision after FNA, the cytologic features were compared with the histopathologic findings. The remaining cases were categorized as one of the three above-mentioned types of mucinous tumors on the basis of cytomorphologic features identified as indicative of subtype among the cases in which a cytologic-histologic correlation was performed. RESULTS Among the 19 cases with cytologic-histologic correlation, 2 cases of serous cystadenoma and 2 cases of gastrointestinal duplication cyst were misdiagnosed on cytologic analysis as low-grade MCN. Among the remaining 15 cases, cytologic features by histologic diagnosis were as follows: ductal adenocarcinoma (n = 4): moderate to high cellularity, mild to moderate background mucin, three-dimensional clusters, high nuclear cytoplasmic ratios, and mild to moderate nuclear membrane irregularities; low-grade MCN (n = 5): mild to moderate cellularity, abundant background mucin, small clusters, and flat sheets of relatively bland glandular cells; mucinous cystadenocarcinoma (n = 1): similar to ductal adenocarcinoma but more abundant background mucin; and IPMT (n = 5): moderate to high cellularity, abundant background mucin, and prominent papillary arrangement of tall columnar cells with mild to moderate nuclear atypia. The remaining 32 cases were categorized based on cytology alone as adenocarcinoma with mucin production (n = 24) and low-grade MCN (n = 8). CONCLUSIONS IPMT and low-grade MCN possess distinctive cytologic features that can be used to diagnose them correctly and distinguish them from one another and from other cystic tumors. Duplication cysts closely mimic low-grade MCN, which can lead to false-positive diagnoses. Because of substantial overlap in cytologic features, mucin-producing ductal adenocarcinoma was unable to be distinguished from mucinous cystadenocarcinoma cytologically.