Aims: To evaluate the results of a novel modular press-þt acetabular cup in primary total hip arthroplasty (THA) for osteoarthrosis (OA) and inßammatory arthritis (IA). Methods: From February 1996 to June 1999 in 324 patients (249 women, 75 men) 355 THA using a novel cup has been carried out. The shell is non-hemispherical on cross-section and has a hydroxyapatite coating on porous titanium for osseointegration. Diagnosis was: osteoarthrosis (OA) 236, developmental dysplasia (DDH) 21, post-traumatic arthrosis 27, avascular necrosis 6, IA 65 Ð mainly rheumatoid arthritis (RA). Average age at operation was 65.8 years. The patients were studied prospectively using Harris Hip Score (HHS), by measuring any radiolucency around the cup and by looking for signs of migration. Results: Median follow-up was 4.5 years. At follow-up, 21 patients had deceased. Revision for deep infection was carried out in 5 hips (3 low-grade infections). No septic loosening occurred with low-grade infection. Recurrent dislocations required revision of 1 cup and exchange of 2 inserts. Only 1 cup in a RA patient with severe superior bone loss became unstable after a fall 4 months postoperatively. Survival with aseptic loosening of the cup as endpoint was 1 in OA and 0.98 in IA. In all 290 patients with 318 THA in follow-up the cup was functioning well, both clinically and radiographically. HHS increased from 44.5 to 90.3. Conclusion: Press-þt þxation using a modern acetabular component is an excellent treatment option in primary total hip arthroplasty for all diagnostic groups.