We evaluated the long-term benefits of steroid injection in 25 shoulders in 20 consecutive patients (average age, 55 years) with primary acromioclavicular arthritis. Minimum follow-up was 5 years. The mean preinjection Constant score (61 points) improved at 6 months to 81 points, (mean difference, 19.36; P < .01). Improvement at 12 months (mean, 86 points) was also significant vs the 6-month score (P = .001). The mean score at 5 years (81 points) was a significant deterioration vs the 12-month score (P = .01) but still a significant improvement vs the preinjection scores (P < .0005). Younger patients had greater improvement in the objective score (range of movement and power; r = -0.47; P = .01), as did women (r = 0.405; P = .05). Local steroid injection is an effective treatment for primary isolated acromioclavicular arthritis. Improvement continues for at least 12 months. The benefit is felt up to 5 years. Pain relief tends to diminish long-term.