Objectives No previous randomised controlled trials had been undertaken investigating the effect of school seating on back pain in 14 to 16 year olds. This study was designed to test the effect of the use of a high-density foam wedge on normal school seating on the intensity of back pain. Design Randomised controlled trial. Setting Suffolk, a predominantly rural county in eastern England. Participants One hundred and eighty-five students with back pain were recruited from 12 schools. Randomisation was stratified by school. The control and intervention groups included 92 and 83 students, respectively. Intervention Following a 1-week baseline observation period, each student in the intervention group was given a wedge to use on their school chairs. Outcome measure The primary outcome measure was pain intensity (numerical rating scale, 0 to 10) recorded in pain diaries over 4 weeks. Random effects models were used to analyse the pain intensity data. Results Ninety-seven students (46 control group, 51 intervention group) completed the trial. For the intervention group, pain intensity was reduced significantly over the 3 weeks of wedge use. The average reduction in pain intensity was estimated to be 0.709 points (95% confidence interval 0.341 to 1.077), representing a 58% reduction in back pain for those in the intervention group. Conclusion Use of a wedge reduced the intensity of back pain significantly, especially in the evenings. The results suggest that further research into the longer-term effect of seating on pain intensity in adolescents should be considered.