This meta-analysis investigated the influence of assessment on the reported effects of problem-based learning (PBL) by applying Sugrue's (1995) model of cognitive components of problem solving. Three levels of the knowledge structure that can be targeted by assessment of problem solving are used as the main independent variables: (a) understanding of concepts, (b) understanding of the principles that link concepts, and (c) linking of concepts and principles to conditions and procedures for application. PBL had the most positive effects when the focal constructs being assessed were at the level of understanding principles that link concepts. The results suggest that the implications of assessment must be considered in examining the effects of problem-based learning and probably in all comparative education research. (Contains 1 note and 3 tables.)