AIM: Several risk factors are associated with heterotopic bone formation following total hip replacement. All these risk factors were defined in cohorts without any treatment against postoperative ectopic bone. The aim of this prospective study was to reveal risk factors for the development of postoperative ossifications in patients who underwent a 7-day course of indomethacin therapy. METHODS: 211 consecutive patients with 217 cementless total hip arthroplasties were included. Patients were given 100 mg indomethacin daily in a 7-day course. RESULTS: Grade 2 or 3 ossifications were observed in 13 (5%) of the male and 3 (2%) of the female Individuals postoperatively (p = 0.0043; odds ratio = 0.45). Older individuals (p = 0.0021; odds ratio = 1.03) as well as patients with primary osteoarthritis (p = 0.0307; odds ratio = 0.28) also showed a higher risk for developing ectopic bone formations. CONCLUSION: With a 7-day course of indomethacin after total hip arthroplasty, male and elderly individuals, as well as patients with primary osteoarthritis, are considered to be at high risk to develop heterotopic ossifications postoperatively. Indomethacin reduces the incidence of postoperative ectopic ossifications, but not the patient's risk.