BACKGROUND. Solid pseudopapillary neoplasm (SPN) and pancreatic endocrine neoplasm (PEN) are uncommon neoplasms that demonstrate characteristic cytologic features. It is also known that both these tumors may share similar morphologic changes. These features not uncommonly pose significant diagnostic challenge for unsuspecting cytopathologists. In the current study, the authors report that recognition of clear cytoplasmic vacuoles in endoscopic ultrasound-guided fine-needle aspiration (EUS-FNA) samples from SPN serves as a useful clue that can distinguish this tumor from PEN. METHODS. The cytologic features from 5 SPN and 20 PEN cases were evaluated. Both Diff-Quik and Papanicolaou stains from these cases were examined. A Fisher exact test of probability was performed to determine differences in the individual cytologic features noted in these 2 tumor types. RESULTS. The results demonstrated that pseudopapillary groups (P = .004); meta-chromatic matrix material (P = .004); nuclear membrane irregularity (P = .004); and large, clear cytoplasmic vacuoles (P = .001) are noted significantly more frequently in SPN. The authors also demonstrated that large, clear cytoplasmic vacuoles can serve as a powerful cytologic clue for the suspicion of SPN over PEN when there is a paucity of papillary groups within the smears. Large, clear cytoplasmic vacuoles, however, were noted only in Diff-Quik-stained smears, but not in Papanicolaou-stained smears. CONCLUSIONS. The results of the current study highlight that large, clear cytoplasmic vacuoles can serve as a critical clue with which to distinguish SPN from PEN in diagnostically challenging cases. © 2008 American Cancer Society.