In nonobese diabetic mice, autoimmune diabetes progresses in an age-linked and gender-dependent manner. Insulitis begins in male and female mice at approximately 1 mo of age; however, 70 to 90% of females, but only 10 to 20% of males, become diabetic by 6 mo. Multiple studies propose that proinflammatory Th1 and immunomodulatory Th2 cytokines impact diabetes pathogenesis, but the role of these cytokines in spontaneous diabetes progression is not yet clear. We used quantitative reverse-transcriptase-coupled PCR to analyze expression of cytokines and APC costimulatory molecules in the islets of 20- to 180-day-old male and female nonobese diabetic littermates, and identified three stages in diabetes progression. At 1 to 2 mo of age, islet-infiltrating T cells displayed a Th1 cytokine bias in females, and a Th2 cytokine bias in males. In females, stage II (2-3 mo of age) was characterized by an increase in islet-infiltrating T cells, APC, and Th1 cytokines, whereas male infiltrates did not increase in size, and Th1 cytokine expression continued to decline during this interval. Islet infiltration reached a plateau (stage III) in 3- to 4-mo-old females, months before overt diabetes onset. Our data imply that Th cytokine expression in early insulitis exerts substantial impact on beta cell destruction and overt diabetes. A clinical implication of our results is that young individuals in the early stages of insulitis are ideal candidates for therapeutic intervention to minimize beta cell destruction and morbidity.