OBJECTIVES: Neck and shoulder pain and disorders were studied among frequent computer users, and the associated effect of mouse and keyboard use was evaluated. METHODS: Technical assistants and machine technicians were followed for 1 year. Questionnaires were sent to 9480 persons (initial response 73%, follow-up response 82%). Computer use information was obtained from the questionnaires. Symptom cases at baseline and follow-up were clinically examined using a standardized clinical protocol. The main outcomes were self-reported pain symptoms in the neck and right shoulder and clinical cases of rotator cuff syndrome, tension neck syndrome, and neck-shoulder pain with pressure tenderness. RESULTS: The prevalence of moderate-to-severe pain in the neck and right shoulder was 4.1% and 3.4%, respectively, and the 1-year incidence for no or minor baseline symptoms was 1.5% and 1.9%, respectively. At baseline, the prevalence rate ratio (PRR) for neck pain was 1.7 [95% confidence interval (95% CI) 1.1-2.6] for mouse use >25 hours/week, that for right shoulder pain increased from 1.6 (95% CI 1.1-2.4) for 15-19 hours/week to 2.5 (95% CI 1.4-4.3) for >30 hours/week of mouse use, and that for tension neck syndrome increased from 3.5 (95% CI 1.0-12) for 25-29 hours/week to 4.7 (95% CI 1.2-18) for >30 hours/week of mouse use . The relative risk (RR) for new neck pain was 1.8 (95% CI 0.8-3.9) for keyboard use 15 hours/week or more and increased to 2.4 (95% CI 0.8-6.8) for ≥30 hours/week or more. New right-shoulder pain symptoms were associated with mouse use >20 hours/week (RR 1.9, 95% CI 1.0-3.5, and RR 3.3, 95% CI 1.2-8.9) and with keyboard use >15 hours/week (RR 2.2, 95% CI 1.0-4.9). CONCLUSIONS: Mouse use is associated with an increased risk of moderate-to-severe pain in the neck and right shoulder, and an association with tension neck syndrome is possible.