Premature uterine contractions were treated by intravenous beta-mimetics in 190 patients during a 2-year period. History and physical examination were directed toward identification of patients with cardiac problems. Unexpected cardiac pathology was discovered in 14 patients, all of whom exhibited severe and continuous nausea, retrosternal pain, or dyspnea during beta-mimetic administration. Treatment was immediately discontinued in the presence of S-T depression, supraventricular tachycardia, nonspecific T wave changes, and sinus tachycardia with right axis deviation. Further investigation revealed obstructive cardiac myopathy in one case and atrial septal defect in another. Such changes might be identified earlier by more extensive screening procedures (such as electrocardiogram) before drug administration. Administration of beta-mimetic agents may uncover previously unexpected cardiac pathology. Continuation of ritodrine in such cases is contraindicated and potentially hazardous.