The validity of the 1991 Finnish Medical Birth Registry data was assessed, with special emphasis on the effects of changes made to the data collection form in 1990. Data abstracted from medical records for all births occurring in 49 hospitals during a five-day sample period (n = 865) were compared to the register information. Good or satisfactory validity was found for 32 of 33 variables, when minor error was tolerated in variables with continuous scales. For diagnoses and procedures, recorded in check-box format, satisfactory validity was found for 10 of 45 variables. Validity could not be assessed for 18 variables because of insufficient number of cases (13 items) or definition problems (5 items). When the results were compared to a 1987 data quality study, many of the variables that had been changed to the check-box format showed improvement in validity. In addition, in some cases a small change in question alternatives or instructions caused a noticeable change in validity.