We investigated the effectiveness of arthroscopic decompression in stage Ⅱ subacromial impingement after long term follow up. Arthroscopic subacromial decompression was done in 104 consecutive patients who had stage Ⅱ sub­acromial impingement. After average of 8.4 years follow up, the final results were as following; 57 shoulders(55%) in excellent, 25(24%) in good, 16(15%) in fair and 6(6%) in poor. All parameters-pain, function, muscle strength and motion-were improved significantly(p<0.00l). Rotator cuff tear was developed in 10 shoulders after decompression. Among them, 8 shoulders had unfavorable results including two poor. In 6 failures, two had rotator cuff tear, three had recurrence of impingement with degenerative change and reflex sympathetic dystrophy was developed in one. Reoperations were done in 4 shoulders. Improvement of impingement symptoms was maintained in the most of patients(79%) after long term follow up. Arthroscopic decompression surgery was very effective means for stage Ⅱ impingement syndrome.