Enlargement of any bursa in or around the popliteal fossa (most commonly the gastrocnemio-semimembranosus (GS) bursa) is called a Baker cyst (BC). Common clinical manifestations of BCs are swelling, mass, pain or stiffness, usually worsening with activity. There may be swelling and tightness or pain behind the knee when walking. However, the majority of these cysts are asymptomatic. They can be detected incidentally in the general population but are more commonly found in patients with osteoarthritis of the knee. In previous studies, aspiration or corticosteroid treatment was frequently used to treat baker's cysts in patients with osteoarthritis and meniscal or ligamentous injuries. However, there is no previous study in the literature showing the efficacy of PRP injection in baker's cyst. In our study, we aimed to compare the efficacy of cyst aspiration and PRP injection into the cyst on pain, function and cyst size compared to cyst aspiration.