We have conducted a prospective double blind randomized and placebo controlled clinical study in 20 patients with shoulder pain syndrome caused by supraspinatus tendinitis to determine whether transdermal nitroglycerin (NTG) has analgesic action in this condition. In a randomized manner we used a 5-mg NTG (Nitroplast) patch per day over 3 days or similar placebo patches applied in the most painful area. Patients were evaluated before treatment was initiated and after 24 and 48 h. The assessment was made blindly by the same clinical investigator. The follow-up showed a significant decrease in intensity of pain at 24 h (7.05 +/- 0.4 to 4.5 +/- 0.5) and 48 h (2 +/- 0.3) in the NTG group (P < 0.003). No changes were observed in the placebo group. The mean pain duration, activity of the extremity and hours of sleep also improved in the NTG group, with no significant modification in the placebo group. Two patients experienced headache as a side effect 24 h after treatment was started. Patients in the NTG group remained free of symptoms when they were assessed 15 days later. We conclude that NTG is useful in the treatment of shoulder pain syndrome caused by supraspinatus tendinitis and that this treatment could be a useful approach to the management of this common disturbance and probably also in other tendon musculoskeletal disorders.