While the management and prognosis of colorectal cancer are largely dependent on clinical features such as tumor stage, there is considerable variation in treatment and outcome not explained by traditional prognostic factors. To guide efforts by researchers and health-care providers to improve quality of care, we review studies of variation in treatment and outcome by patient and provider characteristics. Surgeon expertise and case volume are associated with improved tumor control, although surgeon and hospital factors are not associated consistently with perioperative mortality or long-term survival. Some studies indicate that patients are less likely to undergo permanent colostomy if they are treated by high-volume surgeons and hospitals. Differences in treatment and outcome of patients managed by health maintenance organizations or fee-for-service providers have not generally been found. Older patients are less likely to receive adjuvant therapy after surgery, even after adjustment for comorbid illness. In the United States, black patients with colorectal cancer receive less aggressive therapy and are more likely to die of this disease than white patients, but cancer-specific survival differences are reduced or eliminated when black patients receive comparable treatment. Patients of low socioeconomic status (SES) have worse survival than those of higher SES, although the reasons for this discrepancy are not well understood. Variations in treatment may arise from inadequate physician knowledge of practice guidelines, treatment decisions based on unmeasured clinical factors, or patient preferences. To improve quality of care for colorectal cancer, a better understanding of mechanisms underlying associations between patient and provider characteristics and outcomes is required.