One hundred and twenty consecutive patients with significant coronary artery disease, normal left ventricular systolic function, and coronary collaterals were compared with 111 patients with the same characteristics but with no collaterals. No significant differences were found between the two groups in hypertension, diabetes mellitus, and smoking. The left ventricular end diastolic pressure was 16.4+/-7 in the study group and 16.9+/-6.9 in the controls (NS). Significantly more diseased vessels were observed in the study group than in the control group (2.1+/-0.6 versus 1.7+/-0.6, p=<0.001). One hundred and one totally occluded vessels were found in the study group but only two in the control group. The richest collateral supply was to the right coronary artery: 94 sources to 85 diseased vessels (111%) including 66 sources to 52 totally occluded arteries (127%); to the left anterior descending: 59 sources to 89 diseased vessels (66%) including 37 sources to 33 totally occluded arteries (112%). The poorest supply was to the left circumflex: 17 sources to 69 diseased vessels (25%), including nine sources to 16 totally occluded arteries (56%). No collaterals were observed in 14 totally occluded vessels in the study group and in two of the controls, while the systolic function at rest was still normal. It is suggested that coronary collaterals are important in preserving left ventricular systolic and diastolic performance at least at rest. Not readily visible collaterals may also prevent systolic dysfunction.