Little is known about the validity of self-reported colorectal cancer screening. To date, few published studies have validated all four screening modalities per recommended guidelines or included a general population-based sample, and none has assessed validity over time and by intervention condition. To estimate the validity of self-reported screening, a random sample of 200 adults, ages > or =50 years, was selected from those completing annual surveys on screening behavior as part of an intervention study. Approximately 60% of the validation sample authorized medical record review. Sensitivity, specificity, and positive and negative predictive values were calculated for baseline and year 1 follow-up reports for each test and for overall screening adherence. Sensitivity at baseline ranged from 86.9% (flexible sigmoidoscopy) to 100% (colonoscopy). Sensitivity at follow-up was slightly lower. Adjusting for validity measures, the sample overreported screening prevalence at baseline for each of the four modalities. At follow-up, overreporting was greatest for fecal occult blood test (13.0%). Overreporting across intervention conditions was highest for fecal occult blood test (10.8% for control; 24.8% for the most intense intervention) and overall screening adherence (10.9% for control; 14.3% for the most intense intervention). Sensitivity and specificity of self-reported colorectal cancer screening compared with medical records were high; however, adjusting self-reported screening rates based on relative error rates reduced screening prevalence estimates. Those exposed to more intense interventions to modify screening behavior seemed more likely to overestimate their screening rates compared with those who were not exposed.