BACKGROUND Many physiological processes are subjected to circadian variation. We tested if circadian changes of skin perfusion can be detected by measuring laser Doppler flux (LDF) in the foot and if such variations are different in patients with peripheral arterial occlusive disease (PAOD). METHODS LDF was recorded hourly during a 24-hour period in 10 healthy subjects and in 19 PAOD patients, 9 presenting with claudication (stage II according to Fontaine, ankle artery pressures (AP) 80 +/- 11 mmHg) and 10 with skin necrosis (stage IV, AP 42 +/- 29 mmHg). RESULTS Circadian variability of LDF was significantly reduced in stage IV patients compared both with stage II patients and controls (p < 0.01). An increase in LDF could be observed early in the afternoon and, more pronounced, during the night, occurring to the same extent in healthy and PAOD patients in stage II (p < 0.001). In stage IV the increase in LDF was not significant and a decrease was observed in two patients. LDF and local skin temperature were correlated in controls and stage II patients (mean r = 0.69 +/- 0.12 and 0.7 +/- 0.14, respectively), but not in stage IV (mean r = 0.23 +/- 0.18). A further limitation of LDF variability and nocturnal increase was seen in the presence of diabetes. CONCLUSIONS Circadian variations of skin blood flow are comparable in controls and patients with claudication but markedly reduced in patients with severe PAOD presenting with ulcers, in whom skin perfusion seems uncoupled from temperature regulation, possibly due to local factors causing continuous arteriolar vasodilation.