This review examines current evidence for baseline functional impairment and changes with therapy in patients with peripheral arterial disease (PAD)--ranging from patients without claudication or critical limb ischemia (CLI) but other exertional leg symptoms (erroneously referred to as asymptomatic in the Fontaine classification system), to patients with claudication and those with CLI. The review points out that the status of functional outcomes research is markedly different in focus and development in the different levels of disease severity--paradoxically less studied in the more severe CLI population than in patients with claudication, for example.