The aim of this study was to investigate effects of biofeedback training on trapezius activity and rest (gaps) during occupational computer work. A randomized controlled trial with 164 computer workers was performed. Two groups working with computer mouse more than 50% (n = 64) and less than 25% (n = 49) of their work time performed five sessions with unilateral electromyographical (EMG) biofeedback from the dominant trapezius during computer work. A third group working with computer mouse more than 50% of their work time (n = 51) served as controls. Bipolar EMG from the bilateral upper trapezius muscles during normal computer work was recorded. Changes in discomfort/pain were not recorded. The biofeedback training reduced activity (P < 0.05), and increased the frequency of short (P < 0.05) and long (P < 0.05) gaps, and the relative rest time (P < 0.05) of the trapezius during computer work. By improving trapezius inactivity during computer work, biofeedback training may have the potential to prevent trapezius myalgia in computer workers.