Hypothesis development is a complex cognitive activity, but one that is critical as a means of reducing uncertainty during ill-structured problem solving. In this study, we examined the effect of metacognitive scaffolds in strengthening hypothesis development. We also examined the influence of hypothesis development on young adolescents' problem-solving performance. Data was collected from sixth-grade students (N=172) using a computer-supported problem-based learning environment, "Animal Investigator". The findings of the study indicated that participants using metacognitive scaffolds developed significantly better hypotheses and that hypothesis-development performance was predictive of solution-development performance. This article discusses further educational implications of the findings and future research. (Contains 3 tables and 1 figure.)