OBJECTIVE. To evaluate the reliability of maternal disease registration in the Medical Birth Registry of Norway (MBRN). STUDY DESIGN. VALIDATION STUDY. Setting. The two nationwide population-based registries, the Norwegian Prescription Database (NorPD) and the MBRN, were linked using a unique personal identification number. Population. Pregnant women (n=108,489), pregnancies (lasting more than 22 weeks) starting at March 30, 2004 onwards and ending before January 1, 2007, registered in MBRN. Main outcome measures. Registrations of maternal diagnoses (diabetes, asthma, and epilepsy) in MBRN compared to their dispensal of prescribed medicines as an indication of disease. The sensitivity, specificity, and positive and negative predictive values of diabetes, epilepsy, and asthma in the MBRN were calculated using information from NorPD as the 'gold standard.' METHODS. Validation study comparing maternal diagnoses in MBRN with prescriptions of drugs and reimbursement codes registered in NorPD. Results. The sensitivity for the diagnosis of diabetes (any type) and diabetes type 1 in MBRN were estimated to be 72% (95% confidence interval: 69-74) and 90% (86-93%). For epilepsy and asthma, the sensitivity of MBRN was estimated to be 74% (69-78) and 51% (49-52), respectively. For asthma, the sensitivity increased to over 70% when it was restricted to individuals with more serious disease. CONCLUSIONS. The sensitivity of the registration of type 1 diabetes was 90%, while the sensitivity for any type of diabetes was lower and similar to the sensitivity of the registration of epilepsy. The registration of asthma overall was 51%, but considerably higher for more serious asthma. Except for diabetes type 1, the medical disease diagnoses in the MBRN should be dealt with carefully.