The clinical, biochemical, and neuropathological findings in two neonates with molybdenum-cofactor deficiency presenting with convulsions are reported from the Academic Medical Center, Amsterdam, The Netherlands. Patient 1 was admitted at day two with feeding problems, jitteriness, an abnormal cry, apneic spells, and partial and generalized seizures. Head circumference, weight and length were above the 97th percentile. EEGs showed a burst suppression pattern. CT revealed diffuse hypodense ischemic changes. Plasma and urinary cysteine were decreased, and urine sulphite, S-sulphocysteine, taurine, and thiosulphate were increased. Purine analysis showed elevated xanthine and hypoxanthine in the urine, while uric acid was very low. Seizures were refractory, and the infant died on the 10th day. Cultured fibroblasts from a skin biopsy showed absent sulfite oxidase activity. Autopsy findings were meningeal fibrosis, loss of cortical neurons, gliosis and cystic lysis of white matter. Patient 2 was admitted at age 4 days with feeding difficulties, hypertonia, jitteriness, opisthotonus, and high-pitched cry. Generalized and partial tonic-clonic seizures were resistant to treatment. EEG was a multifocal epileptic pattern. An initial diagnosis of postanoxic encephalopathy was changed to molybdenum-cofactor deficiency at 3 years, on reexamination prompted by the revelation of parental consanguinity. Clinically deteriorated, he had spastic tetraplegia, intractable epilepsy, and characteristic metabolic changes. He died 8 months later. Diagnosis was confirmed by liver biopsy and sulfite oxidase and xanthine oxidase analyses. Lens dislocation, a frequent feature of the syndrome, was absent. [1]