To clarify factors related to the QT interval prolongation produced by probucol, multivariate analysis was applied to clinical and laboratory data retrospectively obtained from 89 patients with hypercholesterolaemia, who had taken probucol for more than 3 months. The corrected QT interval (QTc) increased from 0.410s before treatment to 0.431 s after the administration of probucol; the total cholesterol level decreased from 267 mg.dl-1 to 212 mg.dl-1. None of the patients demonstrated new arrhythmia. The QTc after probucol was independently correlated with sex, serum albumin level and baseline QTc. Changes in QTc after probucol were independently correlated with the presence of ischaemic heart disease, baseline QTc, and a change in the total cholesterol level. The results suggest that a prolonged QTc is likely to appear in female patients, and in patients with a long baseline QTc or with a low serum albumin. It is also suggested that marked lengthening of the QTc is likely to occur in patients with ischaemic heart disease or with a short baseline QTc. Probucol can be used safely in patients with hypercholesterolaemia, but ECG monitoring may be necessary, especially in female patients, as well as in those with hypoalbuminaemia or with ischaemic heart disease.