The purpose of this study was to analyse secondary school students' (N = 16) computer-supported collaborative mathematical problem solving. The problem addressed in the study was: What kinds of metacognitive processes appear during computer-supported collaborative learning in mathematics? Another aim of the study was to consider the applicability of networked learning in mathematics. The network-based learning environment Knowledge Forum (KF) was used to support students' collaborative problem solving. The data consist of 188 posted computer notes, portfolio material such as notebooks, and observations. The computer notes were analysed through three stages of qualitative content analysis. The three stages were "content analysis of computer notes" in mathematical problem solving, "content analysis of mathematical problem solving activity" and "content analysis of the students' metacognitive activity". The results of the content analysis illustrate how networked discussions mediated mathematical knowledge and students' questions, while the mathematical problem solving activity shows that the students co-regulate their thinking. The results of the content analysis of the students' metacognitive activity revealed that the students use metacognitive knowledge and make metacognitive judgments and perform monitoring during networked discussions. In conclusion, the results of this study demonstrate that working with the networked technology contributes to the students' use of their mathematical knowledge and stimulates them into making their thinking visible. The findings also show some metacognitive activity in the students' computer-supported collaborative problem solving in mathematics.