The major predictors of left ventricular function after coronary artery occlusion were assessed in 108 consecutive patients who had complete occlusion of the left anterior descending artery as the only important lesion demonstrated at angiography between June 1978 and June 1983. A scoring system was used to identify regional damage on left ventriculograms. Forty two patients were classified as having good left ventricular function and 66 as having varying degrees of impairment. Apart from a history of myocardial infarction, the only variables discriminating between those with good and those with impaired left ventricular function were the area of distribution of the artery beyond the occlusion and cigarette smoking. Hypertension, hypercholesterolaemia, family history of vascular disease, diabetes, obesity, duration of angina, age, and presence of identifiable collaterals were not discriminators. Smoking was itself significantly associated with a history of infarction; but after controlling for this, smoking exerted a significant additional effect on the amount of left ventricular damage. It is concluded that smoking is not only a risk factor for myocardial infarction in patients with single left anterior descending artery occlusion, but that it is also a major factor in determining the extent of associated left ventricular damage.