The purpose of this study was to investigate the relationship between the contributions students make to the problem-based tutorial group process as observed by their peers, self-study time and achievement. To that end, the Maastricht Peer Activity Rating Scale was administered to students participating in Problem-Based Learning tutorial groups. With this rating scale students had to rate the constructive, collaborative and motivational activities of their peers within the tutorial group. In addition, time spent on self-study was measured with a self-estimation method and achievement was measured with a unit test and a group assignment. A causal model of these variables was developed, in which the three types of activities were assumed to affect time spent on self-study, which would in turn affect unit test scores and group assignment scores. A structural equation modeling analysis indicated acceptable model fit. Especially apparent was the evidence for the causal relations between a student's constructive activities and his/her unit test score and between a student's collaborative activities and the group assignment score. On the other hand, time spent on self-study was not affected by the students' contributions, nor did it have an effect on the unit test score. These results suggest that there are indeed causal relations between a student's contributions to the tutorial group process and achievement. (Contains 2 tables and 2 figures.)