A well but cyanotic newborn was found to have a mutant gamma-globin chain, leading to a functionally abnormal fetal hemoglobin. A single amino acid substitution was found in a site consistent with known adult M hemoglobins. This patient showed no clinical evidence of cyanosis at 5 weeks of age as gamma-chain synthesis was replaced by beta-chain synthesis. A sibling born 20 months later was also cyanotic and the same mutant hemoglobin was found.