The present invention deals with a process for polymerizing ethylene in the presence of an olefin polymerization catalyst comprising titanium, magnesium and halogen in at least one polymerization stage where ethylene is polymerized in slurry, the process comprising treating the polymerization catalyst in a pre-treatment step by polymerizing an olefin on the polymerization catalyst so that the ratio of the weight of the olefin polymer to the weight of the original solid catalyst component is from 0.1 to 10 g/g and using the pre-treated catalyst in the production of ultra-high molecular weight polyethylene.