OBJECTIVE: The blanket term 'repetitive strain injury' (RSI) covers a wide variety of work-related clinical syndromes, most of which are localized lesions. However, some patients complain of diffuse forearm pain, a clinically distinct form of RSI, the aetiology of which is unknown. METHODS: Using Doppler ultrasound, we measured the vascular responses to muscular work in the radial artery in 13 patients with bilateral diffuse forearm pain, seven with unilateral diffuse pain and 19 controls with localized arm pain. RESULTS: We found that in diffuse forearm pain the radial artery is relatively constricted compared to the controls and fails to vasodilate with exercise, which suggests that diffuse forearm pain may be due to physiological claudication of the working forearm muscle. CONCLUSION: A possible explanation is inhibition of local endothelial nitric oxide function, and this may be an unusual secondary, but self-perpetuating, pain condition which can follow other more specific, but chronic, arm pain syndromes in susceptible individuals.