To evaluate an effectiveness of a single intra-articular injection of a new cross-linked hyaluronic acid by measuring thickness of the quadriceps muscle and femoral intercondylar cartilage in addition to subjective assessment. There were 49 patients with gonarthrosis (Kellgren-Lawrence grade II-III) who underwent a single intra-articular injection of Flexotron Cross into the knee joint with higher grade of arthrosis. The scores were assessed at baseline, after 1, 3 and 6 months using: 1) visual analogue scale (VAS) of pain syndrome, 2) Western Ontario and McMaster University Osteoarthritis Index (WOMAC), 3) Lequesne Index, 4) duration of standing on one leg, 5) thigh circumference, 6) thickness of quadriceps muscle and femoral intercondylar cartilage according to ultrasound data. A 6-month follow-up comprised 46 patients. HA administration significantly improved all parameters (p <0.001). VAS and WOMAC pain scores were significantly improved after 1, 3 and 6 months (p <0.01). Thickness of the quadriceps muscle and femoral intercondylar cartilage was increased after 3 and 6 months (p <0.05). Lequesne Index, duration of standing on one leg and thigh circumference were also increased in 6 months after HA administration (p <0.01). Flexotron Cross is effective subjectively and objectively for at least 6 months and safe for the treatment of gonarthrosis.