This study used CT analysis to determine the rotational alignment of 39 painful and 26 painless fixed-bearing total knee replacements (TKRs) from a cohort of 740 NexGen Legacy posterior-stabilised and cruciate-retaining prostheses implanted between May 1996 and August 2003. The mean rotation of the tibial component was 4.3 degrees of internal rotation (25.4 degrees internal to 13.9 degrees external rotation) in the painful group and 2.2 degrees of external rotation (8.5 degrees internal to 18.2 degrees external rotation) in the painfree group (p = 0.024). In the painful group 17 tibial components were internally rotated more than 9 degrees compared with none in the painfree group (p < 0.001). Additionally, six femoral components in the painful group were internally rotated more than 6 degrees compared with none in the painfree group (p = 0.017). External rotational errors were not found to be associated with pain. Overall, 22 (56.4%) of the painful TKRs had internal rotational errors involving the femoral, the tibial or both components. It is estimated that at least 4.6% of all our TKRs have been implanted with significant internal rotational errors.