Myocardial lesions are frequently seen at necropsy after fatal carbon monoxide poisoning. Clinically, while there have been numerous reports of chest pain and electrocardiographic changes associated with acute carbon monoxide poisoning, other evidence for left ventricular abnormality has not been reported. The echocardiographic findings in five cases of non-fatal poisoning are presented here. Abnormal left ventricular wall motion was shown by echocardiography in 3 cases. Motion returned to normal in 2 of the 3 in follow-up tracings. Echocardiograms on 3 of the 5 patients showed mitral valve prolapse. Though the mitral valve prolapse may have been present before the poisoning, the reported high incidence of papillary muscle lesions in fatal cases suggests a possible relation of the prolapse to the effects of the carbon monoxide poisoning.