We report on a scaled-up mathematics intervention in preschool classes (children age six) in Sweden. In the intervention, teachers at seven schools in four municipalities implemented the Thinking, Reasoning and Counting in Preschool Class (TRC) teacher guide in their teaching of 254 students. Results are compared to a previous randomized experimental small-scale intervention based on TRC showing a significant positive effect on students' learning. Pre- and post-test scores for the present scaled-up intervention indicate even higher learning gains compared to the original small-scale intervention. Low-performing students were given extra tutoring time in the scaled-up intervention, but the learning gains for this group cannot be shown to be significantly higher compared to the treatment group in the small-scale study.