Background: A rise in the incidence of thyroid cancer has been reported in several countries, and the increase is only seen in the papillary type. Increased detection due to higher resolution ultrasound and fine needle aspiration has been proposed as the explanation, recent registry studies however question this assumption. Methods: National, unselected, prospective cohort study of 1350 papillary thyroid cancer patients in Denmark from 1996 to 2008. Objective: To analyze changes in incidence by time and to identify factors which might influence detection rate. Results: A rise in incidence is seen with age standardized ratios increasing from 1.43 per 100.000 per year in 1996 to 2.16 per 100.000 per year in 2008. The median age at presentation was 46 years and median tumor size was 18mm. Male/female ratio was 1/2.9. By dichotomizing the material in a time period before and after the 30th of June 2001, no significant change in the proportion of diagnosed tumors smaller than 1 or 2cm was found, and 42.8% of the rise in incidence was explained by tumors larger than 2cm. No significant change in diagnostic use of ultrasound or fine needle aspiration was found, and a significant change toward more extensive thyroid surgery could not be confirmed. Conclusion: This study shows a significant rise in incidence of papillary thyroid carcinoma in Denmark from 1996 to 2008, which is not explained by increased use of preoperative diagnostic modalities. Other reasons need to be considered.