Objective: Low energy lasers are widely used to treat a variety of musculoskeletal conditions. The aim of this clinical study is to determine the action of the IR diode laser 904 nm pulsed on pain reduction therapy. Summary Background Data: With respect to pain, has been shown the Low power density laser increases the endorphin synthesis in the dorsal posterior horn of the spinal cord stopping the production of bradykinin and serotonin. Besides laser causes local vasodilatation of the capillaries and an improved circulation of drainage liquids in interstitial space causing an analgesic effect. Additionally, laser interferes in the cytochines (TNF-α, interleukin-1 and interleukin-6) that drive inflammation in the arthritis and are secreted from CD4 e T cells. Methods: Treatment was carried out on 555 cases and 525 patients (322 women and 203 men) in the period between 1987 and 2002. The patients, whose age ranged from 25 to 70, with a mean age of 45 years, were suffering from rheumatic, degenerative and traumatic pathologies. The majority of the patients had been seen by orthopaedists and rheumatologists and had undergone x-ray, ultrasound scanning, Tac, RM examination. All patients had received drug-based treatment and/or physiotherapy with poor results. Two thirds were experiencing acute symptomatic pain, while the others presented a chronic pathology with recurrent crises. We used a pulsed IR diode laser, GaAs 904 nm, maximum power 60 W, frequency impulse 1300 Hz, pulsed duration 200 nanoseconds; peak power per pulse 27W; maximal energy density: 9J/cm2; total number of Joules per treatment session: 10-75J/cm2, chronic 12-90J/cm2. Average number of applications: 12; maximum number of applications: 20. Results: In the evaluation of the results the following parameters have been considered: disappearance of spontaneous and induced pain (Likert scale, Rolland Morris disability scale, dynamometer). The pathologies treated were osteoarthritis in general, epicondylitis, tendonitis, periarthritis and neurutis, metacarpalgia, metatatarsalgia, Dupuytren's Disease. The summary of the results have been classified in very good = 44%, good = 52.8%, poor = 2.7%, nil = 0.36%, worse = 0%. Conclusions: Treatment with 904 nm IR diode laser interrupts the "circulus vitiosus" of the origin of the different pains (neuropathic, metabolic, traumatic, toxic, immune-mediated, ischaemic) and its development. LLLT brings a normal physiological condition back to the tissue, thus myofascial pain relief, decreases tension and substantially reduces the symptoms as well as improves the quality of life of the patients. More than that, we have discovered an acceleration of the healing process, early return to work and sport competition.