The aim of the current study was to reveal whether 7 days of indomethacin treatment sufficiently prevents heterotopic ossification after cementless total hip arthroplasty. One group received indomethacin for 14 days (n = 102), and the second for 7 days (n = 99) after cementless total hip arthroplasty. At followup 1 year postoperatively, the average Harris Hip Score was 91 points in the 14-day treatment group and 89 points in the 7-day treatment group. The incidence of heterotopic ossification as outlined by Brooker was similar in both groups. Ninety-six patients in the 14-day treatment group had heterotopic ossification Grades 0 or 1, and 6 patients had Grade II heterotopic ossification; whereas in the 7-day treatment group, 95 patients had Grades 0 or 1 heterotopic ossification and 4 patients had Grade II ossification. None of the patients had Grades III or IV heterotopic ossification. In the 14-day treatment group, headache, dizziness, or gastritic disorders develop in 10 patients, and in the 7-day treatment group, 7 patients had these effects. This study shows that treatment with 100 mg indomethacin daily for 7 days is not significantly different than 14 days of treatment for the prevention of formation of severe heterotopic ossification after cementless hip arthroplasty.