BACKGROUND Patients with Raynaud's phenomenon due to systemic sclerosis exhibit a functional microangiopathy with endothelial cell damage. The aim of this study was to assess differences in the so-called nutritive and thermoregulative skin blood flow and to obtain further information by Fourier transformation and by nonlinear analysis of laser Doppler flux (LDF) time series. METHODS A local cold stress test was performed in 10 patients and 10 age- and sex-matched healthy controls. Significant differences were detected between nutritive blood flow in the nailfold capillaries, assessed by capillary red blood cell velocity (CBV), and thermoregulatory blood flow, which was synchronously assessed by LDF. RESULTS CBV was reduced during cooling, the drop being significantly larger in the patients than in the controls. In contrast, there was no significant difference in the decrease of LDF during cooling. The difference between the fall in CBV values and that in LDF was significantly more pronounced in patients. Further analysis of the LDF frequency spectrum by fast Fourier transformation revealed a significantly greater decrease in amplitude at the heart frequency level in healthy volunteers. A further analysis of the LDF signal revealed significant differences at rest and after cooling in fractal dimensions, suggesting an increased complexity of LDF signals in patients. CONCLUSIONS The patient's increased sensitivity towards local cold cn be observed best at the capillary level. But the changes of LDF in the frequency spectrum during cooling as calculated by fast Fourier transformation revealed significant differences. In addition differences in fractal dimensions of LDF time series suggest that an analysis of nonlinear dynamics may be a promising approach.