Purpose: The purpose of our study is to examine early radiographic findings and clinical signs of failure of the microstructure surface HA-coated acetabular cup. Material and methods: This study included 41 revisions of 204 THA with hydroxyapatite-coated acetabular cup from April 1992 to November 1996. Radiolucent line around cups, change in the cup angle and osteolysis were evaluated in serial radiographs. Acetabular cup was defined as loosening if any movement occurred at the bone and socket interface by manual rotation and extension force intraoperatively or migration of more than 2 mm and change of the cup angle by more than 5 degrees in radiographs. Results: The radiolucent line was observed in 12 of the 41 hips at an average of 55.4 months postoperatively and was most commonly located in zone III. All of the 12 hips showing radiolucent lines were classified as loosening by radiographically in 8 cases and intraoperatively in 4 cases. Radiographic loosening occurred at an average of 28.8 months after the appearance of a radiolucent line. Inguinal pain was noted in 18 of the 19 cases revised for acetabular cup loosening. Conclusion: The most important radiographic finding for the early diagnosis of loosening was progressive radiolucent lines, which occurred most frequently in zone 3.