Adequate ligament balancing has a tremendous impact on successful total knee arthroplasty. In case of instability, severely disabling symptoms require revision surgery. Here we present a case of early total knee arthroplasty failure due to secondary valgus laxity, which was successfully treated with medial collateral ligament (MCL) reconstruction. For anatomical MCL reconstruction, a flattened semitendinosus autograft was used to reconstruct the superficial medial collateral and the posterior oblique ligament.