Objective. To investigate the effect of workplace neck/shoulder strength training with and without regular supervision on neck/shoulder pain and headache among office workers. Method. A 20-week cluster randomized controlled trial among 351 office workers was randomized into three groups: two training groups with the same total amount of planned exercises three times per week (1) with supervision (3WS) throughout the intervention period, (2) with minimal supervision (3MS) only initially, and (3) a reference group (REF). Main outcome is self-reported pain intensity in neck and shoulder (scale 0-9) and headache (scale 0-10). Results. Intention-to-treat analyses showed a significant decrease in neck pain intensity the last 7 days in 3MS compared with REF: -0.5 ± 0.2 (P