GRAY SCALE B-scan ultrasonography is accurate in identifying and assessing morphologic features of popliteal artery aneurysms. Eight patients with suspected popliteal aneurysms had 12 aneurysms demonstrated by ultrasonography, including two not visualized by arteriography due to proximal occlusive disease and three covert aneurysms opposite a contralateral palpable popliteal mass. Half the patients had bilateral aneurysms demonstrated by ultrasonography. Popliteal artery ultrasonography is useful to: (1) evaluate suspected aneurysms in patients who cannot or should not be subjected to invasive arteriographic studies or operation; (2) confirm or refute equivocal diagnoses of aneurysms generated by arteriographic studies or physical examination; (3) detect contralateral clinically covert aneurysms or small aneurysms in patients with generalized extrapopliteal aneurysmal diseases; (4) delineate the extent of intraaneurysmal thrombus and determine exact aneurysm size in all dimensions; and (5) define a thrombosed aneurysm or one noe opacified because of limited contrast material passing through proximally obstructed arteries. The latter two uses of ultrasonography represent significant advantages over arteriography.