BACKGROUND Gender differences have been documented among patients diagnosed with colorectal cancer (CRC). It is still not clear, however, how these differences have changed over the past 30 years and if these differences vary by geographic areas. We examined trends in CRC incidence between 1975 and 2006. METHODS The study population consisted of 373,956 patients ≥40 years diagnosed with malignant CRC between 1975 and 2006 who resided in one of the nine Surveillance, Epidemiology and End Results (SEER) regions of the United States. Age-adjusted incidence rates over time were reported by gender, race, CRC subsite, stage, and SEER region. RESULTS Overall, CRC was diagnosed in roughly equal numbers of men (187,973) and women (185,983). Men had significantly higher age-adjusted CRC incidence rates across all categories of age, race, tumor subsite, stage, and SEER region. Gender differences in CRC age-adjusted incidence rates widened slightly from 1975 to 1988, reached a peak in 1985-1988, and have narrowed over time since 1990. The largest gap and decline in CRC incidence rates between men and women were observed among those ≥80 years (p<0.001), followed by those 70-79 and then 60-69 years. Gender differences in CRC incidence rates for the 40-49 and 50-59 age categories were small and increased only slightly over time (p=0.003). CONCLUSIONS Higher CRC age-adjusted incidence among men than among women has persisted over the past 30 years. Although gender differences narrowed in the population ≥60 years, especially from 1990 to 2006, gender gaps, albeit small ones, in those younger than 60 increased over time. Future studies may need to examine the factors associated with these differences and explore ways to narrow the gender gap.