Two related studies are reported about the (lack of) activation of real-world knowledge in elementary school students' understanding and solution of school arithmetic word problems. In the first study a set of word problems was collectively administered to 75 fifth-graders during a typical mathematics lesson. Half of the problems were standard items that could be unambiguously solved by applying the most obvious arithmetic operations with the given numbers, while the other half were problematic items for which the appropriate mathematical model was less obvious and indisputable. In the second study a similar problem set was individually administered to 30 fifth-graders in an attempt to collect more fine-grained data about the strength and awareness of conceptions and beliefs concerning the role of real-world knowledge in understanding and solving word problems. The results of both studies support the hypothesis that students tend to exclude common-sense knowledge and realistic considerations during the solution of arithmetic word problems. Furthermore, this tendency seems to be rooted in implicit but resistant beliefs about the role of real-world knowledge in mathematical modeling of school word problems. Contains 20 references. (Author/MKR)