Seventy two consecutive patients with severe isolated aortic regurgitation were evaluated by preoperative echocardiographic and angiographic assessment of the aortic root. Biopsy specimens of the aortic wall were taken at operation. Two major groups of patients were found: those with cusp derangement but normal aortic roots and those with normal cusps but dilated aortic roots. Of the 42 cases of abnormal cusps, 20 were rheumatic, 15 were infective, and six were bicuspid. One patient had a tear in an otherwise normal cusp. Of the 30 cases of abnormal roots but normal cusps, six had inflammatory changes (syphilis, Reiter's disease, giant cell aortitis) and 24 had root dilatation caused by non-inflammatory destruction of elastic laminae. Echocardiographic measurement of the aorta at the level of the top of the commissures predicted the findings at pathology. In 37 of 39 patients with cusp disease the measurement was less than 37 mm. In 27 of 33 patients with root disease the measurement was greater than or equal to 37 mm. This difference was statistically significant. There was no difference in the sizes of the prosthesis used in each group, suggesting that it was the diameter of the junction of the aorta with the sinuses rather than the junction of the sinuses with the ventricle that was important in aortic regurgitation. Clinical progression in patients with non-inflammatory aortic root disease is slower than in patients with infective disease but faster than in those with rheumatic cusp disease.