Quantitatively estimating functional reserve of blood supply to the legs in patients with peripheral arterial occlusive disease (PAOD) remains a clinical issue. This study was designed to investigate the regional blood supply to the legs in PAOD patients during exercise by use of thallium 201 (²⁰¹Tl) whole-body imaging in comparison with transcu taneous PO₂ (tcPO₂) measurement. Thirty-three patients with PAOD and 10 subjects without PAOD (control) performed an incremental cycle ergometry (CE), while tcPO₂ was continuously registered on the involved calf. In the last minute of exercise, 2 mCi of ²⁰¹Tl was injected intravenously and the ²⁰¹Tl whole-body images were taken immediately (stress) and four hours (redistrib ution) following stress with a dual-head camera system. Regional blood supply (RBS) (%) was calculated from the geometric mean counts of the region of interest divided by the total counts of the whole body. The performance of PAOD patients was reduced in doing CE, and tcPO₂ fell distinctly in PAOD patients (from 51 to 19 mmHg) whereas it increased in controls (from 57 to 67 mmHg). The RBS in PAOD patients was obviously reduced in comparison with that of controls. While in controls the RBS of the calf (3.1%) at stress did not differ from that at redistribution (3.4%), in PAOD patients the redistribution RBS (2.8%) increased as compared with that of stress (1.5%). There was a hyperbolic relationship between stress RBS of the calf and the velocity of tcPO₂ fall in PAOD patients during exercise test (velocity of tcPO₂ fall = -0.032 + 0.39/RBS, r2 = 0.54, P < 0.05). In conclusion, the RBS determined by ²⁰¹Tl whole-body imaging is comparable to the tcPO ₂ measurement in differentiating patients with PAOD from subjects without PAOD during exercise. Regional ²⁰¹Tl uptake reflects regional blood supply in PAOD patients. There is a hyperbolic relationship between the RBS derived from ²⁰¹Tl whole-body imaging and tcPO₂ in PAOD patients during exercise, implying that in a critical ischemia the lower the RBS is, the more steeply the tcPO₂ decreases.