Nonketotic hyperglycinemia was diagnosed in identical twins with lethargy and respiratory failure in the neonatal period. Therapy with strychnine (0.32 mg/kg/day) resulted in great reductions in CSF and plasma glycine levels and improvement in muscle tone, respiration, and ability to suck. Myoclonic seizures were partially controlled by therapy with clonazepam. Higher dosages of strychnine (up to 2.0 mg/kg/day) were needed to counteract the increased lethargy following administration of clonazepam. At 5 months of age, the twins' developmental performance remained below the 1-month level despite adequate somatic growth. The twins died suddenly of status epilepticus at 6 1/2 months of age.