A series of patients with meningococcal infections have been studied and divided in two groups: Group I patients with meningococcal sepsis and group II, those with meningococcal meningitis. Patients in group I presented with more severe encephalopathy, shock, DIC and acute systemic complications. Both groups showed a marked hypoaminoacidemia compared with normal controls (other than for the sulfur containing amino acids and phenylalanine). The concentration of aromatic and basic amino acids, the phenylalanine/tyrosine ratio, the transaminase levels and the negative nitrogen balance were higher in group I patients. The ratio of branched chain to aromatic amino acids was lower in group I. All these differences were statistically significant. The close association between the metabolic derangements and clinical manifestations may help in the understanding of several physiopathological aspects of meningococcal infections.