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Modular hip prostheses are increasing in variety and utilization. Component stability, high endurance limit, and minimal particulate debris generation are critical for long-term clinical success. The purpose of this study was to characterize the fatigue response and evaluate the in vitro potential for component motion and wear of the S-ROM¿, a Ti-6Al-4V hip prothesis with a modular design based on a Morse taper connection. A fatigue jib was designed to simulate fixation of the device at the sleeve-bone interface only with distal support mainly against the lateral endosteal cortex. Two series of tests were performed in air at room temperature: one with direct vertical loading (to produce high bending moments in the coronal plane) and one with a compounding loading angle directed at 15 degrees out-of-plane (to include torsional physiological loads). Applied loads using a servohydraulic test machine ranged from 5 x BW (body weight) to 9 x BW (1 x BW = 73 kg, approximately 160 lb) at 10 Hz on an Instron apparatus. No mechanical failures were observed on the 11-mm size stems below 6 x BW for in-plane vertical loading, and at or below 7 x BW for out-of-plane loading. Using displacement monitoring with a sensitivity of 35 mum, no measurable slippage or relative motion was detected between the stem and sleeve when they were properly assembled. Examination of the contact areas with scanning electron microscopy releaved random surface modification (an indication of fretting or burnishing) with occasional evidence of transfer of material between stem and sleeve.(ABSTRACT TRUNCATED AT 250 WORDS)