The C57BL/6J mice are inbred strains and develop the metabolic syndrome of obesity, hyperinsulinemia, hyperglycemia, and hypertension,when fed a high-fat diet. These features are similar to those observed in the human metabolic syndrome. This article examined the effect of fat-enriched (FE) diet on the pattern of distribution of insulin-, glucagon-, somatostatin-, and pancreatic polypeptide (PP)-positive cells in the pancreatic islets of C57BL/6J mice using immunohistochemical methods. Insulin-immunoreactive cells were observed in both the peripheral and central regions of the islets of Langerhans in both FE- and control diet-fed mice. The percentage distribution of insulin-positive cells was similar in FE (83.5 ± 6.4) compared to control diet-fed C57BL/6J mice (83.8 ± 6.5). Glucagon-containing cells were discerned in the periphery of pancreatic islets in both FE- and control diet-fed mice. The percentage distribution of glucagonwas not statistically different in mice fed with FE (9.9 ± 2.7) compared to control diet (11.3 ± 4.9). Somatostatin-positive cells were seen in the outer part of the islet of Langerhans and constitute 12.1% (±6.3) and 10% (±5.5) of pancreatic islet cells in FE-and control diet-fed mice, respectively. PP-immunoreactive cells were observed in the peripheral region of the pancreatic islets of both FE- and control diet-fed mice. The percentage distribution of PP-positive cells was significantly (2.0 ± 1.2) lower compared to control (5.1 ± 2.4). In conclusion, the number of PP is significantly reduced in FE diet-fed mice and may play a role in the pathogenesis of diet-induced metabolic syndrome in C57BL/6J mice.