Female nonobese diabetic (NOD) mice more frequently develop autoimmune diabetes than NOD males. Orchidectomy of the latter aggravates insulitis and diabetes. Because clear differences in immune function have not been observed between prediabetic females and males, before or after castration, we hypothesized that sex-related differences in diabetes incidence are related to target organ-specific actions of sex steroids. Previously, we showed that prediabetic NOD females develop hyperinsulinemia and subsequently mega-islets. Infiltration of the first inflammatory leukocytes is predominantly associated with these mega-islets. Here, we determined the relationship between sex hormones, mega-islet formation, and infiltrating cells in NOD and nonobese diabetic/severe combined immune-deficient (NODscid) mice. Mega-islet formation was reduced in NOD males compared with NOD females, and orchidectomy increased it, indicating a relationship between androgen levels and mega-islet formation. Moreover, enhanced mega-islet formation in castrated NOD males was associated with increased numbers of infiltrating leukocytes. Castrated NODscid males also exhibited increased mega-islet formation and dendritic cell infiltration, indicating that lymphocytes are not required for castration-induced effects. In conclusion, we show that androgens influence pancreatic islets and autoimmune infiltration in NOD and NODscid mice. This suggests that the gender difference in diabetes incidence in NOD mice is related to target organ-specific androgen effects.