A higher incidence of cor pulmonale exists in the pediatric age group, especially among infants, than was previously suspected. The majority of cases come from the patients whose lung disease is associated with fibrocystic disease of the pancreas; and because these children now tend to live longer, with improved antibiotic therapy, the chances of seeing this complication are increased. A large proportion (over 70% in this series) of infants and children with this type of lung disease show cor pulmonale at autopsy. This means, statistically speaking, that fibrocystic disease has a high rank among the precursors of pulmonary heart disease in the general population. Other severe lung diseases of childhood, tuberculosis and bronchiectasis, for example, rarely produce cor pulmonale, and the asthmatic patients usually escape entirely. Approximately half the patients with cor pulmonale have terminal decompensation and die in severe right-sided cardiac failure. This may occur suddenly, only a few days after the first signs of chest disease, or may come on gradually after many years. The duration of disease bears an inverse ratio to the severity of the initial insult. Massive overloading of the pulmonary circulation by blood-borne foreign bodies may cause immediate death, and fibrocystic lung disease may produce such abrupt pulmonary hypertension that the patient survives only a few days. But most infants and children are able to compensate after the initial strain and show a gradual progression to the decompensated state over a period of years. The diagnosis of cor pulmonale assumes greatest interest at the infant level. It is now clear that otherwise mysterious episodes of infant cardiac failure have been on the basis of pulmonary disease. In an older child with extensive pulmonary disease and clinical evidence of a failing heart, the presence of proved fibrocystic disease makes cor pulmonale a likely postmortem finding. An infant with the same combination of signs presents a more difficult problem in differential diagnosis since his critical condition often makes direct analysis of pancreatic enzymes hazardous, and stool analysis for trypsin activity may be unreliable under these circumstances. Once the diagnosis is established, however, all effort should be directed toward clearing the chest of infection, since therapy of the secondary cardiac involvement is generally unsuccessful.