Background: Increases in thyroid papillary carcinoma incidence rates have largely been attributed to heightened medical surveillance and improved diagnostics.Weexamined papillary carcinoma incidence in an equal-access health care system by demographics that are related to incidence. Methods: Incidence rates during 1990-2004 among white and black individuals aged 20 to 49 years in the military, and the general U.S. population were compared using data from the Department of Defense's Automated Central Tumor Registry and the National Cancer Institute's Surveillance Epidemiology and End Results (SEER-9) program. Results: Incidence was significantly higher in the military than in the general population among white women [incidence rate ratio (IRR) = 1.42; 95% confidence interval (CI), 1.25-1.61], black women (IRR = 2.31; 95% CI, 1.70-2.99), and black men (IRR = 1.69, 95% CI, 1.10-2.50). Among whites, differences between the two populations were confined to rates of localized tumors (women: IRR=1.73, 95% CI, 1.47-2.00; men: IRR=1.51, 95% CI, 1.30-1.75), which may partially be due to variation in staging classification. Amongwhite women, rates were significantly higher in the military regardless of tumor size and rates rose significantly over time both for tumors ≤ 2 cm (military: IRR = 1.64, 95% CI, 1.18-2.28; general population: IRR = 1.55, 95% CI, 1.45-1.66) and > 2 cm (military: IRR = 1.74, 95% CI, 1.07-2.81; general population: IRR = 1.48, 95% CI, 1.27-1.72). Among white men, rates increased significantly only in the general population. Incidence also varied by military service branch. Conclusions: Heightened medical surveillance does not appear to fully explain the differences between the two populations or the temporal increases in either population. Impact: These findings suggest the importance of future research into thyroid cancer etiology.