In our observational study, we performed the clinical and functional examination, analyzed imagistic and histological findings and evaluated the correlation between previous aspects in patients with primary knee osteoarthritis (PKOA) and Baker's cyst (BC). The correlations were made to better understanding of BC in patients with PKOA and optimal choosing for treatment. Seventy patients with painful PKOA (ACR criteria) and BC were assessed. We evaluated knee pain using a 100 mm VAS and functional status using the pain, stiffness and functional subscales of WOMAC index. All patients were imagistic examined (ultrasonography and MRI). Thirty-eight patients with PKOA and simple BC respond to conservative treatments. Thirty-two patients with PKOA and complex BC need surgical removal (arthroscopic decompression ± open excision in larger cysts). For these patients, it is performed histological assessment. Any medical team that manages a PKOA patient with BC may develop the treatment plan based upon not only the size of BC, symptoms and other associated conditions but also on the WOMAC scoring and complex anatomic and histological data about BC.