The article discusses the patient management with knee osteoarthritis according to the ESCEO 2019 algorithm. The focus is on the second stage in the persistence of osteoarthritis signs. The article pays attention to the means based on hyaluronic acid for intra-articular injection. It also provides the study analysis that proved the anti-inflammatory mechanism of action of hyaluronic acid-based drugs during intraarticular administration, as well as the study results in which histology data indicated that hyaluronic acid prevents cartilage degradation and can promote its regeneration. These researches show that repeated injections of hyaluronic acid are not only effective, but also safe. In the literature, there is evidence that during treatment, the time to endoprosthesis increases, as well as the need for taking NSAIDs and opioids decreases. The article discusses the molecular weight of hyaluronic acid, providing research data confirming the greatest efficacy of hyaluronic acid with medium and high molecular weight. KEYWORDS: osteoarthritis, knee joints, hyaluronic acid, intraarticular injection. FOR CITATION: Filatova Yu.S., Gawert V.R. Intra-articular hyaluronic acid: dependence of the effect on molecular weight. Russian Medical Inquiry. 2021;5(3):156–161. DOI: 10.32364/2587-6821-2021-5-3-156-161.