Laser-Doppler velocimetry is used to visualize the entrance flowpattern of polyethylene melts undergoing an abrupt 14:1 contraction flow through a slit die. Due to the high accuracy of laser-Doppler velocimetry, a quantitative determination of the velocity vectors in the corners of the contraction region is obtained. Experimental data of this kind together with a thorough theological characterization are intended to be a base for numerical simulations. Three low-density polyethylenes were investigated. The entrance flow patterns were measured at different output rates and temperatures. For the three polyethylene melts large secondary flow regions were found in the corners of the die. Very significant differences in vortex size as a function of temperature point out that temperature has a significant effect on secondary flow. Furthermore, it is found that changes of the output rate influence secondary flow, too. For the three polyethylenes of different rheological properties it could be shown that within the accuracy of the measurements the vortex sizes come to lie on one curve if plotted as a function of the Weissenberg number. This curve exhibits a maximum that can qualitatively be related to the strain-hardening behavior of the samples.