This two-part article examines the histologic and morphologic basis for stenotic and purely regurgitant aortic valves. Part I discusses stenotic aortic valves and Part II will discuss causes of purely regurgitant aortic valves. In over 95% of stenotic aortic valves, the etiology is one of three types: congenital (primarily bicuspid), degenerative, or rheumatic. Other rare causes of stenotic aortic valves include active infective endocarditis, homozygous type II hyperlipoproteinemia, and systemic lupus erythematosis. The causes of pure aortic regurgitation are multiple but can be separated into diseases affecting the valve (normal aorta) (infective endocarditis, congenital bicuspid, rheumatic, floppy), diseases affecting the walls of aorta (normal valve) (syphilis, Marfan's, dissection), disease affecting both aorta and valve (abnormal aorta, abnormal valve) (ankylosing spondylitis), and diseases affecting neither aorta nor valve (normal aorta, normal valve) (ventricular septal defect, systemic hypertension). Diseases affecting the aortic valve alone are the most common subgroup of conditions producing pure aortic valve regurgitation.