This paper examines the reactions of 10 elementary teachers to a teaching case used as part of a professional development project's curriculum. Their written reactions to both reading the case and participating in a 2-hour discussion of it suggest that cases may stimulate teachers to think about their own understanding of mathematics, the mathematical thinking of children, and their roles as teachers. In addition, patterns in the data suggest that teachers' reactions to cases are strongly colored by their prior experience with case materials, their abilities to articulate the subtleties of reformed mathematics teaching practice, and where they are in their thinking about mathematics education reform. Contains 17 references. (Author)