The posterior condylar angle is formed by the transepicondylar axis and the tangent line to the posterior condyles. It is an important relationship to determine rotational alignment of the femoral component in total knee arthroplasty. We measured this angle directly in 107 osteoarthritic knees undergoing total knee arthroplasty. The posterior condylar angle was significantly greater in valgus knees than in other osteoarthritic knees. Given the standard deviations and ranges of values noted, the posterior condyles are potentially unreliable references for femoral component rotation in some knees.