UNLABELLED Bone ingrowth promises more durable biologic fixation of megaprostheses. The relative performance of different types of fixation is unknown. We compared the fixation of two forms of biologically fixed femoral components: an intramedullary uncemented press-fit stem (UCS; Group 1, 50 patients) and a Compress((R)) uncemented fixation (CPS; Group 2, 41 patients). In Group 1, the overall Kaplan-Meier prosthetic survival rates were 85% at 5 and 71% at 10 years. Most failures were long-term developments. Aseptic loosening was the primary cause of failure. Stem diameters less than 13.5 mm and a diaphyseal/stem coefficient greater than 2.5 mm were associated with decreased prosthetic survival. In Group 2, the overall rate of CPS survival was 88% at 5 years. Failure of femoral fixation or fracture during the first year was the main reason for revision. Five-year survival rates were similar between the groups and we observed no difference in the functional success of the implants. We found no failures after 1-year followup in Group 2 (CPS). Any difference in prosthetic survival can only be proven by longer-term study or a randomized trial. LEVEL OF EVIDENCE Level III, therapeutic study. See Guidelines for Authors for a complete description of levels of evidence.