Causal Attribution research deals with the explanations people find in situations of success and failure for why things happened the way they did, and the extent of control they feel to have over the situation. Attributing success and failure differently has an impact on our emotions, our motivation, and behavior. However, so far research on computer-related attributions has not answered the question whether different attribution patterns influence system evaluation in usability tests. This question formed the basis for our investigation. Two standardized questionnaires were used to measure users' attribution patterns and users' system evaluations. The usability tests were conducted in our laboratory with N=51 participants. At large, our results suggest that there are notable influences of users' attribution patterns on their evaluation of system quality, especially in situations of success.