Teachers in primary education experience difficulties in teaching word problem solving in their mathematics classes. However, during controlled experiments with a metacognitive computer programme, students' problem-solving skills improved. Also without the supervision of researchers, metacognitive computer programmes can be beneficial in a natural class setting. In this study, we examined the effectiveness of a computer programme in such a setting. During 10 weeks, the Grade 5 students of the experimental condition (n = 280) practised with the programme while also working in their mathematical textbooks. The programme offered metacognitive hints to support them in solving the mathematical problems. The students of the control condition (n = 110) also worked in their textbooks for the 10-week period but had no access to the computer programme. The results indicate that the computer programme is suitable for implementation in practice and beneficial to the students in combination with their mathematics textbooks.