We report an infant with a bronchiolitis-like illness and rapid deterioration who developed a cor pulmonale-like picture with a dilated right ventricle. Urinary organic acid assays established a probable diagnosis of Cbl-C-type methylmalonic aciduria, later confirmed by complementation studies. Despite medical intervention and cyanocobalamin treatment the patient died on his tenth hospital day. Postmortem examination showed the presence of thromboemboli in the pulmonary circulation. We hypothesize that acute cor pulmonale developed in this infant secondary to thromboembolism of his pulmonary circulation. A review of the literature shows that thromboembolism may be a part of this disease process.