Subacromial corticosteroid injection is one of the most frequently used management tools in subacromial impingement syndrome (SIS) despite controversial reports on the efficacy. Our purpose, in this single blinded, randomised and controlled study was to clarify whether the corticosteroid injection provides additional benefit when used with other conservative treatment modalities in 48 patients with stage 2 SIS. The patients were randomly divided into three groups according to the two therapeutic injections applied with a 10-day interval: group 1: 10 cc of 1% lignocaine + 40 mg of methylprednisolone for the first and second injections, group 2: 10 cc of 1% lignocaine + 40 mg of methylprednisolone for the first injection and only 10 cc of 1% lignocaine for the second injection, group 3: only 10 cc of 1% lignocaine for the first and second injections. All the patients were prescribed 500 mg of naproxen sodium to use two times daily, instructed to rest and perform Codman's pendulum exercises during the first 15 days. Shoulder pain during rest, activity, and causing disturbance of sleep was evaluated using a visual analogue scale and shoulder function was investigated by total Constant score and its subsectional parameters which are pain, daily living activities, active range of motion and strength before the therapy and 1 and 3 months after the therapy onset. Significant improvements from the baseline values in all pain and function parameters were observed at the first and second evaluation in all groups. Group 1 patients had more favourably improved values in pain causing sleep disturbance and daily living activity parameters than group 2 and 3 patients only in the 1st month after therapy onset. We found that subacromial corticosteroid injections in the acute or subacute phase of SIS provided additional short-term benefit without any complication when used together with nonsteroidal anti-inflammatory drugs (NSAIDs) and exercise.