Physical activity levels were measured with an accelerometer in a case study including 19 children, from nine to ten years of age, in a Danish primary school. The teachers conducted their teaching in a forest every Thursday from 2000 to 2003. The purpose of this study was to measure the students' activity levels during outdoor learning days in the forest and compare them with a traditional school day on the one hand, and a traditional school day including two physical education lessons, on the other. The mean activity levels were more than twice as high on the forest days measured on two occasions. Expressed in percentages, winter (2000) measurements were 106% (range 14-194%), and summer (2001) measurements were 113% (range 41-224%) higher compared to traditional school days. Furthermore, in 2002 the mean activity level measured on a day including two physical education lessons was equal to a day in an outdoor environment. In conclusion, this case study demonstrates a significantly higher level of physical activity when indoor and outdoor learning contexts are combined. From a physical health perspective outdoor learning is recommended in the folkeskole. (Contains 7 figures and 2 tables.)