Pyruvate carboxylase deficiency results in congenital lactic acidosis. We report the significant finding in a child with infantile spasms controlled with adrenocorticotrophin hormone (ACTH) but who then developed severe lactic acidosis; pyruvate carboxylase deficiency was subsequently diagnosed. Blood lactate, pyruvate, and alanine levels were elevated, as well as cerebrospinal fluid alanine. Plasma alanine concentration was doubled by ACTH therapy. Fibroblasts contained extremely low pyruvate carboxylase activity. The patient died at 12 weeks of age after recurrent episodes of profound acidosis. At autopsy, the brain manifested cystic degeneration and demyelination. Pyruvate carboxylase deficiency is associated with neonatal onset of acidosis, delayed development, seizures, hypotonia, recurrent profound acidosis, and early death. The dramatic rise in plasma alanine content coincident with ACTH therapy suggest that ACTH played a role in precipitating the catastrophic metabolic acidosis.