Studies have reported an increasing incidence of thyroid cancer since 1980. One possible explanation for this trend is increased detection through more widespread and aggressive use of ultrasound and image-guided biopsy. Increases resulting from increased detection are most likely to involve small primary tumors rather than larger tumors, which often present as palpable thyroid masses. The objective of the current study was to investigate the trends in increasing incidence of differentiated (papillary and follicular) thyroid cancer by size, age, race, and sex. Cases of differentiated thyroid cancer (1988-2005) were analyzed using the National Cancer Institute's Surveillance Epidemiology and End Results (SEER) dataset. Trends in incidence rates of papillary and follicular cancer, race, age, sex, primary tumor size (<1.0 cm, 1.0-2.9 cm, 3.0-3.9 cm, and >4 cm), and SEER stage (localized, regional, distant) were analyzed using joinpoint regression and reported as the annual percentage change (APC). Incidence rates increased for all sizes of tumors. Among men and women of all ages, the highest rate of increase was for primary tumors <1.0 cm among men (1997-2005: APC, 9.9) and women (1988-2005: APC, 8.6). Trends were similar between whites and blacks. Significant increases also were observed for tumors > or =4 cm among men (1988-2005: APC, 3.7) and women (1988-2005: APC, 5.70) and for distant SEER stage disease among men (APC, 3.7) and women (APC, 2.3). The incidence rates of differentiated thyroid cancers of all sizes increased between 1988 and 2005 in both men and women. The increased incidence across all tumor sizes suggested that increased diagnostic scrutiny is not the sole explanation. Other explanations, including environmental influences and molecular pathways, should be investigated.