It is important for our understanding of the pancreatic islets to study whether new islets are able to form in the intact pancreas. We developed a new method to determine the total number and the mean volume of the pancreatic islets, and we used this method to study the expansion of the islet mass in ob/ob mice (n = 8), using ob/+ mice (n = 8) as controls. The total islet volume was increased by a factor of 3.6 in ob/ob mice compared with ob/+ mice, whereas, importantly, the total number of islets did not differ among ob/ob mice and ob/+ mice (3,193 ± 160 islets in ob/ob mice vs. 3,184 ± 142 islets in ob/+ mice, P = 0.97). The coefficient of variation in the volume distribution of islets was equal in the two groups, showing that in ob/ob mice, the existing islets expand their volume by the same proportion, without a net formation of new islets. We suggest that the pancreatic islets should be considered as anatomically such complex structures that islet neogenesis does not spontaneously occur in an intact pancreas. Cells within the existing islets are presumably the most important sources for islet cell hyperplasia during expansion of the total islet mass.