A simulator test was carried out in the hip joint simulator on a metal-on-metal coupling of a modified CoCrMo alloy. The test was run up to 1.9 million cycles and the tribological behaviour of the coupling was documented by means of roundness measurements and scanning electron microscopy. It is seen that the tribological system metal on metal is characterized by mixed friction in the artificial hip joint, thus generating material contacts that are documented by the roundness variation and by the SEM contact zone analysis, showing "three-body" wear of this system. The roundness variations indicate that running-in wear is to be noticed at first with a decreasing tendency over the 1.9 million cycles. Nevertheless, this observation does not allow any prognosis as to the wear rates in the case of a higher number of cycles. It must be taken into consideration that aqua destillata had to be used as a lubricant for technical reasons; the use of serum/Ringer solution might have produced different results. Further investigations in the hip joint simulator and on revised metal-on-metal couplings will be carried out in this respect.