Aims. We conducted a study to identify factors that are prognostic of the outcome of extracorporeal shockwave therapy (ESWT) for calcific tendinitis of the shoulder. Patients and Methods. Since 1998, patients with symptomatic calcific tendinitis of the rotator cuff have been treated with ESWT using an electrohydraulic mode shockwave device. One year after ESWT, patients were grouped according to the level of resorption of calcification. Results. Of 241 symptomatic shoulders, complete resorption (CR) of calcification occurred in 134 (CR group). The remaining 107 shoulders had incomplete resorption (ICR) (ICR group). Gartner type I calcification was most common (64.5%) in the ICR group. The mean duration of symptoms before ESWT was significantly longer in the ICR group. Overall, 81% of the CR group and 23.4% of the ICR group were symptom free. There was a strong relationship between subsidence of symptoms and remission of calcification. Poor prognosis was significantly related to Gartner type I calcification, calcification extent > 15 mm and duration of symptoms > 11 months. Conclusion. Patients with calcific tendinitis of the shoulder who have the factors identified for a poor outcome after ESWT should undergo a different procedure. Cite this article: Bone Joint J 2017;99-B:1643–50