At base line, the average percentage of stenosis caused by the worst lesion in each of nine proximal segments was 34 percent. On average, after 2 1/2 years of conventional therapy, this index of stenosis increased by 2.1 percentage points. By contrast, it decreased by 0.7 point during treatment with lovastatin and colestipol and by 0.9 with niacin and colestipol (P for trend <0.003). Thus, at the end of the study, on average, these nine lesions were almost 3 percentage points less severe among patients treated intensively rather than conventionally. This difference represents almost 1/10 of the amount of disease present at base line (34 percent stenosis). The minimum diameter, an alternative index of the severity of disease, in the nine proximal lesions averaged 1.91 mm for all patients. It decreased (worsened) by 0.050 mm with conventional therapy but increased (improved) by 0.012 mm with lovastatin and colestipol and by 0.035 with niacin and colestipol (P for trend <0.01).