EUROCAT Northern Netherlands registers children born with a birth defect in the Northern Netherlands. Data used to be collected via a notification form, which is filled out by the notifier and, if necessary, completed by the general practitioner. To increase the amount of information, EUROCAT started using new methodology in July 1997. The new procedure consists of three additional steps. Firstly, a parental questionnaire with 43 questions concerning pregnancy and medical history is sent to the parents. Also, the pharmacist is approached to provide information on the drugs that were dispensed to the mother in the period from three months before until the end of the pregnancy. The last step is a telephone interview with the mother. In this study the old and new method are compared with respect to response, quality and quantity of the data. Of the 198 parental questionnaires included in this study, 179 (90.4%) were returned. The pharmacists returned 173 out of 179 requests for information (96.6%). The parental information is more complete for ethnicity and serum screening. The quality of the drug exposure data is much better using the new methodology. The general practitioner's input is still necessary for specification and verification of the diagnosis.