This study quantified the radiographic changes of osteoarthritic acetabular bone cysts after uncemented total hip arthroplasty (THA). Ten-year follow-up radiographs from 130 primary THAs were reviewed. Forty-one cysts were identified on immediate postoperative radiographs. Two-dimensional cyst size was measured postoperatively and on long-term radiographs. Mean initial cyst size was 1.0 +/- 0.9 cm(2). Four (10%) of the 41 cysts expanded over time. The average increase in cyst size was 5.1 +/- 8.6 cm(2) which equated to a mean size increase of 713%. Twenty-seven cysts (66%) shrank, and the remaining 10 cysts (24%) did not change with time. Because cysts located in zone II, near the dome hole of the cup, were statistically more likely to progress, we hypothesize that these cysts communicated with the joint space via these holes. We propose that placing the acetabular component to seal an osteoarthritic cyst from the joint space during THA can help prevent its subsequent progression and the risk of associated osteolysis.