In the nonobese diabetic (NOD) mouse model for type 1 diabetes, the inflammatory infiltration of islets starts with an influx of dendritic cells (DC) and macrophages (M[phi]) at approximately 4 weeks of age. Around this time, NOD mice show endocrine abnormalities, indicated by a transient hyperinsulinemia that lasts until 8 weeks of age. Subsequently, they develop abnormally large islets of Langerhans, here designated as "mega-islets." NODscid mice, which lack functional lymphocytes, also exhibit transient hyperinsulinemia, but to a lesser extent. First, to determine the role of lymphocytes in the morphological islet abnormalities, we compared 6-week-old (prediabetic) NOD and NODscid females regarding mega-islet development and accumulation of antigen-presenting cells (APC), particularly CD11c⁺ DC and ERMP23⁺ M[phi] . In NODscid mice, early APC infiltration and mega-islets were present, but less marked compared with NOD mice, thus suggesting a role of lymphocytes in mega-islet formation. In both NOD and NODscid mice, the APC infiltration was predominantly found around the mega-islets, suggesting a relationship between both parameters. Second, to analyze the role of [beta]-cell hyperactivity in mega-islet formation, we studied the effect of short-term prophylactic insulin treatment on these parameters. Prophylactic insulin treatment decreased the percentages of mega-islets in both NOD and NODscid mice, indicating that [beta]-cell hyperactivity is also involved in mega-islet formation. In conclusion, mega-islet formation in mice with the NOD genetic background takes place under the influence of both [beta]-cell hyperactivity and leukocytes.