Responses to interview questions were compared for concordance among 492 individuals interviewed more than once in a hospital-based case-control surveillance system in the United States, Canada, and Israel between 1976 and 1982. Reliability of the data was determined using the Kappa statistic and the intraclass correlation coefficient. Reliability was good to excellent for demographic factors, such as birthplace, and for medical conditions/procedures that require hospitalization or continuing medical care, such as hysterectomy. Reliability was fair to good for less serious or less well-defined medical conditions/procedures, such as cystic breast disease, and for current habits, such as daily coffee consumption. Regarding medication use, reliability was poor to fair for drugs taken intermittently, such as aspirin and penicillin, and good to excellent for drugs taken on a regular basis, such as oral contraceptives. As expected, medications were reported more consistently when duration of use was prolonged. The data were also analyzed according to two intervals between interviews (less than 1 year and greater than or equal to 1 year). For most factors, reliability was not materially affected by interval. Where differences were observed, reliability tended to be better when the second interview followed the first by less than 1 year. These results suggest that structured interviews administered to hospital patients by trained personnel can elicit reliable data on demographic and medical history factors.