Blood fluidity is reported to influence capillary perfusion in healthy subjects and especially in patients with different diseases [6,10,29,31]. Up to now there is no simple way to quantify the influence of the different haemorheological parameters (haematocrit, plasma viscosity, erythrocyte aggregation, erythrocyte deformability) on the velocity of blood cells in human capillaries. In general, only a limited correlation can be shown [18]. One reason is the great variability of the diameter of human capillaries described in healthy subjects, with values between 3 and 15 m, and in patientsdepending on the disease -with values up to 50 m (e.g. giant capillaries in patients with scleroderma [33]). This is a very large range with respect to the role of the various haemorheological parameters of blood in microcirculation.The hypothesis of Barras, that plasma viscosity determines the perfusion of microvessels because the absolute capillary haematocrit is about 10-20%, so that blood viscosity in capillaries approaches plasma viscosity seems to be near the truth [4], as long as the capillary diameter is big in relation to the erythrocyte diameter. The work of Lipowsky would seem to confirm this [25]. In the case where the capillary diameter is smaller than the red blood cell diameter, the cells have to be deformed by shear forces before passing through a capillary. In this case the blood flow velocity should additionally depend on the erythrocyte deformability. This fact, which results from a fluid-dynamical point of view, however, is often not seen in patients using capillary microscopy in nailfold capillaries: neighbouring capillaries with different diameters and fed from the same arteriole show the same red blood cell velocity. Therefore we conclude that there is a regulatory mechanism in the capillary network possibly by the microvascular endothelium as described by different groups [8,15,26,34].In an animal model Driessen showed that rigidified erythrocytes slow down the capillary blood flow (up to stasis) only if the blood pressure was lowered below the normotensive state [9]. On the other hand in extreme cases of sickle cell crisis, a dramatic near zero flow in cutaneous capillaries can occur [21,24] because of the extremely reduction of the deformability of the sickled erythrocytes [27] together with their adhesiveness [3].The problem is more complicated in cases of a pathologically elevated erythrocyte aggregation. Under these conditions a massive sludging can be found -firstly described by M.H. Knisely´s group in conjunctival vessels -even in very small cutaneous capillaries [12]. Big erythrocyte aggregates pass very slowly through the capillary while immediately thereafter a single cell perfusion with significantly 1386-0291/16/$35.00