An orthopaedic surgical instrument , comprising: a customized patient-specific surgical instrument comprising: a body having a planar proximal surface and a planar distal surface , a cutting guide slot extending through the body , a pair of first arms extending posteriorly from the body , each arm including a first customized patient-specific negative contour configured to receive a portion of a first corresponding positive contour of one of a patient's femoral condyles , a second arm extending proximally from the body , the second arm having a second customized patient-specific negative contour configured to receive a portion of a second corresponding positive contour of an anterior surface of the patient's femur , and an alignment slot extending through the body and the proximally extending arm and positioned to align with a deepest portion of a trochlear groove of the patient's femur when the customized patient-specific surgical instrument is positioned on the patient's femur , characterized by : the alignment slot extending through the body from an opening extending through the planar distal surface of the body to an opening extending through the planar proximal surface of the body . a customized patient-specific surgical instrument comprising: a body having a planar proximal surface and a planar distal surface , a cutting guide slot extending through the body , a pair of first arms extending posteriorly from the body , each arm including a first customized patient-specific negative contour configured to receive a portion of a first corresponding positive contour of one of a patient's femoral condyles , a second arm extending proximally from the body , the second arm having a second customized patient-specific negative contour configured to receive a portion of a second corresponding positive contour of an anterior surface of the patient's femur , and an alignment slot extending through the body and the proximally extending arm and positioned to align with a deepest portion of a trochlear groove of the patient's femur when the customized patient-specific surgical instrument is positioned on the patient's femur , a body having a planar proximal surface and a planar distal surface , a cutting guide slot extending through the body , a pair of first arms extending posteriorly from the body , each arm including a first customized patient-specific negative contour configured to receive a portion of a first corresponding positive contour of one of a patient's femoral condyles , a second arm extending proximally from the body , the second arm having a second customized patient-specific negative contour configured to receive a portion of a second corresponding positive contour of an anterior surface of the patient's femur , and an alignment slot extending through the body and the proximally extending arm and positioned to align with a deepest portion of a trochlear groove of the patient's femur when the customized patient-specific surgical instrument is positioned on the patient's femur , characterized by : the alignment slot extending through the body from an opening extending through the planar distal surface of the body to an opening extending through the planar proximal surface of the body . <EOS>