Sporadic gastrin-producing neuroendocrine tumors (NETs) of the duodenum present with either Zollinger-Ellison syndrome or unspecific syndromes. Ki-67 scoring in cytopathology is an alternative approach for establishing the gastrinoma grade. Although the majority of NETs, including gastrinomas, occur in the duodenum, most research regarding the Ki-67 index is focused on tumors of pancreatic origin. To the best of our knowledge, there is no study on the Ki-67 index for cytological analysis of duodenal gastrinoma. The current report presents two cases of a 56-year-old man and a 66-year-old woman with NET G1 and G2 gastrinoma, respectively, arising in the duodenal bulb. The present report focused on the differences in nuclear pleomorphism and Ki-67 index between these two cases.