Background: Current guidelines for colorectal cancer screening do not specify the role of age or comorbidity in patient selection. Reported screening practices may thus be influenced by patient, as well as physician, characteristics. Methods: A questionnaire was sent to primary care physicians in 10 states to determine recommendations for fecal occult blood testing (FOBT) and sigmoidoscopy in 4 pairs of clinical vignettes that varied by patient age (65 or 75 years) and comorbidity (none, mild, moderate, and terminal). Results: Among the 884 respondents (50%), most physicians recommended FOBT in all vignettes except those with a terminal illness and fewer than half recommended sigmoidoscopy in any vignette. Physician recommendations were influenced by comorbidity, but one third still recommended FOBT for terminally ill patients and less than 50% recommended sigmoidoscopy for patients with no comorbidity. Differences in recommended screening between vignettes that differed only in patient age were small. Among all subgroups of physicians, at least 30% recommended FOBT in terminally ill patients and no more than 60% recommended sigmoidoscopy in healthy patients. Conclusions: Primary care physicians were much more likely to recommend screening with FOBT than sigmoidoscopy, regardless of patient and practitioner factors. Although physician recommendations were influenced by patient comorbidity and age, they frequently recommended FOBT in clinically inappropriate settings and failed to recommend sigmoidoscopy when appropriate. Broad-based interventions to improve appropriate screening practices may be needed.