Three hundred and seventy four aortic valves which had been surgically removed over the past five years were studied by routine histology. Most patients were male and over the age of 60 years. There were 3.7% bicuspid valves, 16% valves with evidence of past rheumatic fever and 2.1% with endocarditis. A range of pathological lesions was seen including calcification, chondroid and osseous metaplasia, neovascularization, inflammation and cholesterol deposition. A common lesion was a progressive dystrophic calcification of the valve cusps. This was studied and graded in relationship to the concomitant structural damage. There has been no previously published grading system for this type of pathological change in the aortic valve. Our criteria for the four grades of aortic valve lesion are described. Most patients were found to have lesions of Grades 3 and 4.