Sleep apnea (SA) is associated with increased morbidity of the cardiovascular system, the interaction between the disordering of respiratory coordination and cardiovascular regulation being largely unknown. In 64 patients (age: mean = 54.1; range: 35-67 years) with an increased apnea index (AI greater than 10), a cardiac catheterisation investigation was performed to exclude coronary heart disease (CHD) or cardiomyopathy. CHD was excluded in 39 patients, 6 patients had coronary single-vessel disease, 9 patients coronary two-vessel, and 10 three vessel disease. In 10 patients, cardiomyopathy was detected, while high-grade impairment of the left ventricular ejection fraction (greater than 30%) was observed in five patients. With the exception of a single patient, CHD was observed only in patients in the over-fifty age group. Arterial hypertension was seen in 84% of the patients with, and in 69% of the patients without, CHD. The patient groups with and without coronary heart disease did not differ with respect to apnea index, ten minute index, or the average duration of the 30 longest apneic episodes. Anginal complaints, observed in a total of 72% of the patients, were one of the major indications for coronary angiography. These results do not support the assumption that SA is primarily a consequence of underlying cardiac disease, but do indicate that SA must be considered a cardiac risk factor, especially in view of the fact that pronounced nocturnal changes in blood gases and haemodynamics, together with malignant arrhythmias, are found in conjunction with this disturbance of breathing.