Respiratory alkalosis is an early sign of urea cycle disorder. A high level of plasma ammonia will strengthen this suspicion. It is of great importance to transfer the infant as soon as possible to a unit capable of giving specific treatment with Na-benzoate, Na-phenylbutyrate, argininchloride and carglumic acid. The early treatment may also include haemodialysis, which is preferred over peritoneal dialysis or exchange transfusion. We here describe an infant with respiratory alkalosis within the first two days of life and a high plasma level of ammonia (> 700 micromol/L). He did not respond to conventional therapy and died 48 hours after birth in spite of specific treatment. DNA-analysis showed a gene defect in the OTC gene, c.67C >T (p.R23X), a known mutation leading to urea cycle disorder (OTC). It is important to detect carriers among older siblings and to inform the parents of the possibility of prenatal diagnostics.