Recall bias is a major concern in case-control studies in which questionnaire data are used to assess past exposure. The authors conducted a validation substudy within the framework of a parent case-control study on risk factors for acute lymphoblastic leukemia in children aged < or =9 years diagnosed in 1980-1993 in Quebec, Canada. Parental recall bias for two variables was assessed: reported distance from home to power lines compared with measured distance and reported prenatal radiographic examinations compared with hospital medical record data. For reported distance, sensitivity was 62% for a subgroup of cases living in an area in which an excess of childhood acute lymphoblastic leukemia was perceived and was attributed to power lines. However, for other cases, sensitivity (35%) was similar to that measured for hospital controls (36%) and was relatively close to that for population controls (22%). Specificity was high for all groups except cases from the area with a perceived excess. Sensitivity for reported prenatal radiographic examinations was similar for cases (64%) and population controls (71%) but was lower for hospital controls (50%). Results confirm that under special circumstances, such as those resulting from enhanced public concern, parental recall can be differential but otherwise is most often nondifferential, with low sensitivity. Choosing the best type of controls to ensure comparable recall accuracy remains difficult.