Aims: To determine the in vivo movements between the polyethylene and the metal tibial tray in modular fixed bearing TKA. Methods: 16 patients (median age 72) operated with the NexGen TKA were studied. The metal tray of the tibial component was equipped with 5 tantalum markers, and the polyethylene insert with 6 markers. Radiostereometric (RSA) investigation was performed within one week postop., and at 12 months. Change in position of the poly insert in relation to the metal tray between the postop and the 12 months investigation was analyzed. Insert motion index (IMI) = √AP² +ML2 was also calculated where AP is the largest AP-translation and ML the largest ML-translation. Results: The polyethylene component rotated a median 0.55° (0.09° to 1.21°) in relation to the metal tray in the horizontal plane, mostly externally, corresponding to a median IMI of 415 μm (19 to 920 μm). Movements of the polyethylene out of the plane of the tibial tray were very small and generally below the precision of the RSA method. Conclusions: This study shows for the first time that movements do occur in vivo between the polyethylene insert and the metal tray in modular fixed bearing TKA. Almost all movements occur in the horizontal plane of the metal tray and very little movements are detected in directions out of this plane. The magnitudes of IMI found in vivo study are equivalent to those found in vitro in studies of explanted components.