OBJECTIVE: To estimate the risk of adverse birth and neonatal outcome, and miscarriage in women who used sulfamethizole during pregnancy. METHODS: The association between use of sulfamethizole and adverse birth and neonatal outcome was investigated in a case-control and a cohort study in Denmark. We used data from the Prescription Database, the Birth Registry and the Hospital Discharge Registry in North Jutland County to study any association between sulfamethizole use and first recorded miscarriage. The cohort analysis included 3484 women who received a prescription for sulfamethizole from 30 days before conception to date of delivery, and 60175 women who did not use a sulphonamide-containing drug during pregnancy or 30 days before conception. The case-control analysis included 3347 women who had a miscarriage, of whom 90 had taken sulfamethizole, and 22599 primiparous controls who had a live birth. RESULTS: Among women who received prescriptions for sulfamethizole, adjusted odds ratios and 95% confidence intervals for adverse birth outcome were: malformation 1.17 (0.95-1.43); low birth weight 0.69 (0.49-0.98); pre-term birth 1.12 (0.97-1.30); stillbirth 1.02 (0.61-1.68); neonatal jaundice 1.14 (0.38-3.46); and for receiving a prescription for sulfamethizole within 1 week before miscarriage 1.66 (0.92-2.99). CONCLUSIONS: We found no increased risk of congenital malformation, stillbirth or pre-term birth, and no association between use of sulfamethizole late in pregnancy and risk of neonatal jaundice. There was an increased risk of miscarriage after exposure to sulfamethizole during the week before miscarriage, but further studies are needed to evaluate whether this increased risk is causal.