Osteoarthritis of the knee joints and varicose veins of the lower extremities are two common diseases that quite often occur in combination in patients. What pathology is primary? This is an important issue for specialists, since specialized treatment is provided in different surgical departments and often without correction of the venous outflow there is no possibility of surgical treatment of the knee joint. Based on the literature and personal experience, we have established a possible connection between these two diseases with an attempt to determine which of them will be the root cause. The literature analysis was carried out using Elsevier, PubMed, eLibrary, PLOS and Cyberleninka databases. Articles containing the keywords: osteoarthritis, gonarthrosis, varicose veins, lower extremities, knee joint, venous congestion were analyzed. English and Russian full-text articles, literature reviews, systemic reviews, meta-analyses, cohort studies, and traditional reviews with a search depth of no more than 35 years were studied. It has been established that the most widespread theory is one that identifies varicose veins of the lower extremities as the root cause of osteoarthritis of the knee joint. In our opinion, both diseases should not be considered as the root cause. The presence of one pathology may aggravate the clinical picture of another. Having conducted a single-center randomized prospective study in 40 patients with varicose veins of the lower extremities, we found that 24 of them, with clinical manifestations of osteoarthritis of the knee joint, were diagnosed with clinical classes of varicose veins C3–C5 according to CEAP.