A usability evaluation technique, Cooperative Usability Testing with Questions of Understanding (CUT with QU) intended to illuminate users’ ability to understand the content information of an application is proposed. In complex application domains as for instance the eHealth domain, this issue of users’ content understanding is sometimes crucial, and thus should be carefully evaluated. Unfortunately, conventional usability evaluation techniques do not address challenges of content understanding. In a case study within eHealth, specifically the setting of a rehabilitation clinic involving the participation of four physiotherapists and four clients in a period of 3.5 months, it was demonstrated how CUT with QU can complement conventional usability testing and provide insight into users’ challenges with understanding of a new complex eHealth application. More experiments in other complex application domains involving different kinds of users and evaluators are needed before we can tell whether CUT with QU is an effective usability testing technique of wider applicability. Performing CUT with QU is very demanding by drawing heavily on the evaluators’ ability to respond effectively to openings and potential shortcomings in the users’ content understanding. Evaluators need to train this interview/examination process in order to be able to reach a proper insight of the user’s content understanding. If CUT with QU after more research shows to be inadequate, the motivating research question behind this experimental study remains important: How can “content understanding” effectively be brought into usability testing in complex application domains?