Some patients complain of chronic persistent medial knee pain after isolated low-grade injuries of medial collateral ligaments (MCL). These injuries often respond to various conservative treatments but insufficient healing of ligaments is believed to be responsible for symptoms in this group of patients. We report on successful treatment of chronically symptomatic MCL injuries in 3 patients using autologous leukocyte-reduced platelet-rich plasma (PRP) injections. There were 3 men with a mean age of 39.3 years and a mean disease duration of 10 months. Magnetic resonance imaging showed discontinuities of superficial layers and thickness of deep layers in the proximal MCL. After PRP injections, all cases returned to their sport activities at a previous level as without symptoms, and complete healing of proximal ligaments was identified on magnetic resonance images. The outcomes indicated that PRP injections led to successful repair for chronic injuries of MCL in knees.