Incidence rates for colorectal cancer are decreasing in the United States, possibly due to preventative cancer screening. Because these programs target older patients, their beneficial effects may not apply to young patients. The purpose of this study was to compare incidence rates and tumor characteristics of colon and rectal cancers for young versus older patients using a population-based cancer registry. Colon and rectal cancer patients reported in the Surveillance, Epidemiology, and End Results registry (1973-1999) were separately analyzed. Incidence rates over time, stage, and grade were compared for two age groups: young patients (20-40 years, n = 5383) and older patients (60+ years, n = 256,401). For older patients, colon cancer incidence remained stable while rectal cancer incidence decreased 11 per cent to 72.1/100,000 persons (P < 0.05). For the young, colon cancer incidence increased 17 per cent to 2.1 (P < 0.05), and rectal incidence rose 75 per cent to 1.4 (P < 0.05). Young patients had less localized tumors than older patients: colon (25.8% vs. 35.3%, P < 0.001); rectal (38.4% vs. 41.7%, P = 0.005). Young patients also had more poorly differentiated tumors: colon (22.2% vs. 14.7%, P < 0.001); rectal (16.4% vs. 12.3%, P < 0.001). Incidence rates for colon and rectal cancers in young patients are rising, and they have more advanced disease. Although the overall prevalence is low in this population, the increasing incidence suggests health-care providers should have heightened awareness when caring for this population.