Objective: To define diameter at three levels along the popliteal artery and its relation to the inflow arteries in the normal state and in popliteal aneurysms. Methods: The external diameter of the arteries was determined by duplex ultrasound scanning at the common femoral (CFA), superficial femoral artery (SFA), proximal popliteal artery (PPOP), mid-popliteal artery (MPOP), and distal popliteal artery (DPOP). Examinations were performed in 104 healthy men and 100 women. In addition, patients were screened for the presence of popliteal aneurysms (diameter >10 mm). Findings in healthy male subjects were compared with those with popliteal aneurysms. Results: Mean arterial diameters in normal men were larger than in women, but the SFA/CFA ratio was smaller in women (0.74 ± 0.08 vs 0.78 ± 0.09; P < .001) and the MPOP/SFA ratio was larger (0.98 ± 0.11 vs 0.94 ± 0.12; P = .001). In both genders, normal popliteal artery diameter was not uniform throughout its length, with PPOP and MPOP being nearly identical, and DPOP was smaller. MPOP diameter correlated most closely with SFA diameter (r = 0.51; P < .001) and less with height, weight and body surface area (r = 0.2 to 0.3) and was not associated with age or the presence of hypertension. In 27 men with 45 patent, fusiform popliteal aneurysms (10 to 44 mm) the site of maximal dilatation was in the region of the MPOP in 39 cases and near the PPOP in only 6 cases. The DPOP was never the largest segment and only in one case was it >10 mm. Arterial diameter in aneurysm patients was larger than normal at all levels but was greatest near the MPOP level (15.7 ± 6.9). Popliteal-to-SFA diameter ratios were increased in the aneurysm group at all three levels but were greatest at the MPOP level (1.85 vs 0.94, P < .001). Comparing 15 popliteal aneurysms >20 mm with smaller ones, distal popliteal artery changed to the least extent but did increase in diameter (6.1 ± 1.2 vs 7.0 ± 1.4, P < .04). In larger aneurysms the MPOP/SFA ratio increased from 1.54 to 2.5 (P < .001). Conclusion: The diameter of the normal popliteal artery is not uniform throughout its length. In popliteal aneurysms, the region of the MPOP is most commonly the largest diameter. The MPOP/SFA ratio is greater than normal in popliteal aneurysms and increases in larger aneurysms. DPOP does dilate but to a lesser extent then PPOP and MPOP, making endovascular repair anatomically feasible in most popliteal aneurysms.