In the experiment presented in this paper the Elaboration Likelihood Model (ELM), a social psychological theory of persuasion, was applied to explain why users sometimes agree with the incorrect advice of an expert system. Subjects who always agreed with the expert system's incorrect advice (n = 36) experienced less mental effort, scored lower on recall questions, and evaluated the cases as being easier than subjects who disagreed once or more with the expert system (n = 35). These results show that subjects who agreed with the expert system hardly studied the advice but just trusted the expert system. This is in agreement with the ELM. The experiment also covers an investigation into the factors that moderate user agreement. The results have serious implications for the use of expert systems.