Objectives: To evaluate the effectiveness of high-intensity laser therapy on shoulder pain and function in subacromial impingement syndrome. Design: Clinical controlled trial with alternate allocation. Setting: Hospital Department of Rehabilitation. Subjects: A total of 46 participants with subacromial impingement syndrome. Intervention: Participants were sequence allocated to an intervention group (high-intensity laser therapy + exercise therapy) and control group (sham-laser + exercise therapy) and received 15 sessions (five days a week during three weeks). Main measures: Patiens were evaluated at baseline, after 15 sessions, and at one month and at three months after completing the intervention. The main outcome variables were pain and functionality as measured by visual analogue scale; pressure pain threshold; Shoulder Pain and Disability Index; Constant-Murley Score; and QuickDASH. Secondary outcomes were number of sessions at discharge and drug use. Results: A total of 21 patients in high-intensity laser therapy group (56.7 ± 8.9 years) and 22 patients in sham-laser group (61.3 ± 8.9 years) concluded the study. Visual analogue scale (cm) at baseline, one-month, and three-months were 6.2 ± 0.5, 3 ± 2.6, and 2.6 ± 2.4 for the control group and 5.4 ± 1.5, 3.6 ± 1.3, and 1.8 ± 1.7 for experimental group, respectively. Shoulder Pain and Disability Index (points) at baseline, one-month, and three-monts were 51.8 ± 16.1, 16.3 ± 16.1, and 13.6 ± 17.1 in the control group and 41.8 ± 20.6, 20.5 ± 19.7, 11 ± 14.5 in experimental group, respectively. No differences were found between groups (P > 0.05). Conclusion: The effect of high-intensity laser therapy plus exercise is not higher than exercise alone to reduce pain and improve functionality in patients with subacromial syndrome.