The fixation of screw-fixed, porous-coated acetabular prostheses was studied during 2 years in 21 patients (22 hips) using repeated roentgen stereophotogrammetric measurements. Migration of the center of the prostheses and/or tilting were recorded in five cups after 6 months. Two years after operation, migration was recorded in three, migration and rotation in six, and only rotation in three cups. Bone quality, prosthetic position, and immediate postoperative radiographic appearance of the prosthesis-bone interface did not correlate with the occurrence of migration. Narrow radiolucent zones had developed in half of the patients after 2 years and was associated with proximal migration. Micromovements of screw-fixed, press-fit cups seem to start later compared with previously presented results from cemented acetabular prosthesis.