Flow behaviour of human seminal fluid was studied with a rotational viscometer (Rotoviso RV 100 with low shear sensor CV 100 made by Haake Karlsruhe) regarding normozoospermia, oligozoospermia, azoospermia and initially prolongated liquefaction time. Measurement is done according to DIN 53018 (procedure) and DIN 53019 (evaluation) and viscosity is given in millipascal seconds (mPas). At a shear rate of 90 s-1 the absolute viscosity n is 7.86 +/- 5.51 mPas (n = 104); without the initial viscous ejaculate (n = 23) viscosity n at 90 s-1 was 5.55 +/- 1.71 mPas (n = 81). Within the group of viscous ejaculates the average viscosity is distinctly higher than the one in the group of normal ejaculates. In both groups there is no correlation between number of spermatozoa and viscosity and motility and viscosity. A reason for the different flow behaviour is not yet found.