Many victims of the sudden infant death syndrome (SIDS) have abnormally heavy cardiac right ventricles. The degree of this abnormality is directly proportional to: a) the mass of muscle about small pulmonary arteries, b) the amount of brown fat retention about adrenal glands, and c) the presence of hepatic erythropoiesis. The pulmonary arterial abnormality is probably the result of chronic alveolar hypoventilation, while brown fat retention and hepatic erythropoiesis are likely consequences of chronic hypoxemia. These abnormalities are found in both SIDS victims who die with and those who die without mild respiratory tract infections. However, there are some differences between the two SIDS groups. Infected victims die at an older age and have smaller thymus glands and larger spleens; there is a greater proportion of males in the infected victims than in the noninfected victims.