With the aim to investigate microvascular endothelial function in chronic kidney disease (CKD) patients on conservative treatment, skin blood flowmotion (SBF) was explored by spectral Fourier analysis of skin forearm laser Doppler tracing, registered before and following forearm ischemia in 32 III to V stage CKD patients (23 males, mean age: 52+/-12 years), without diabetes or cardiovascular disease, and in 32 age and sex matched healthy subjects. The power spectral density (PSD) of the 0.009-1.6 Hz total spectrum SBF, as well as of five sub-intervals, each of them related to endothelial (0.009-0.02 Hz), sympathetic (0.02-0.06 Hz), myogenic (0.06-0.2 Hz), respiratory (0.2-0.6 Hz) or cardiac (0.6-1.6 Hz) activity, was measured in PU(2)/Hz (PU=perfusion unit; 1 PU=10 mV). Under basal conditions CKD patients and controls did not differ in skin perfusion or in PSD of total spectrum SBF, as well as of each of the five subintervals considered. No substantial difference was also observed in skin post-ischemic hyperemia between patients and controls. A significant post-ischemic increase in the normalized value of endothelial sub-interval was observed in controls (p<0.05, GLM ANOVA analysis of variance), but not in CKD patients. A lower per cent increase in absolute PSD value of endothelial sub-interval was also observed in CKD patients compared to controls (185+/-98 % vs 279+/-243 %, p<0.05). The post-ischemic per cent increase in absolute PSD of endothelial sub-interval was negatively related to the systolic blood pressure (r=-0.45, p<0.01), to the mean arterial blood pressure (r=-0.40, p<0.05) and to the PTH serum levels (r=-0.38, p<0.05) in CKD patients. The blunted post-ischemic increase of the endothelial SBF sub-interval can be considered an early sign of microvascular endothelial dysfunction in the CKD studied patients. Arterial hypertension seems to be the main factor related to this SBF abnormality, together with the hormonal CKD related abnormalities.