OBJECTIVE: To evaluate magnitude and mode of acute load to failure of the Zurich Cementless acetabular cup prosthesis in cadaveric specimens with and without 50% dorsal acetabular rim loss. STUDY DESIGN: In vitro mechanical study. SAMPLE POPULATION: Cadaveric hemipelves of adult dogs (n = 8). METHODS: Each pair of hemipelves was prepared by dissection of surrounding musculature and implantation of a Zurich Cementless acetabular cup prosthesis. One hemipelvis had the dorsal rim left intact (group 1) and the contralateral hemipelvis had 50% of the dorsal rim excised (group 2). Each hemipelvis underwent acute load to failure with an axial load applied through a prosthetic femoral head. Load at failure was compared between hemipelves with and without dorsal rim loss with a paired t-test; P < .05 was considered significant. RESULTS: Mean failure load was not significantly different between group 1 (3,713 ± 362 N) and group 2 (3,640 ± 751 N; P = .8). Bone fracture (n = 6), ventroversion of the cup (1), and absolute failure unreached at 6,000 N (1) occurred in group 1 and bone fracture (6), ventroversion of cup (1), and cup loosening (1) occurred in group 2. CONCLUSIONS: Zurich Cementless acetabular cup stability does not appear to be compromised by 50% acetabular rim loss at normal physiologic weight bearing loads. Thus, for this system, modifying procedures such as augmentation of the dorsal acetabular rim or deeper reaming for acetabular bed preparation may not be necessary with up to 50% dorsal rim loss with the Zurich Cementless acetabular cup.