313 patients with subacromial impingement were randomized to either arthroscopic subacromial decompression, arthroscopic irrigation alone, or no treatment. Patients were primarily assessed for function, on the Oxford Shoulder Score, after 6 and 12 months. Comparing the two arthroscopic interventions to each other, no significant difference in functional scores were observed between the arthroscopic subacromial decompression group and the arthroscopic irrigation alone group at 6 or 12 months. Comparing the two arthroscopic interventions to no treatment, while statistically significant greater function scores were observed for both arthroscopy groups compared to no treatment, the differences were deemed not likely to be clinically relevant as they did not meet the minimum clinically important difference of 4.5pts for the Oxford Shoulder Score.