Osteosarcoma is the most common malignant bone tumor. It is localized in areas of rapid bone turnover, most commonly in the distal femur and proximal tibia, predominantly in young individuals between 10 and 25 years of age, when bone growth is intense. Modern treatment consists of neoadjuvant therapy, complete surgical resection and adjuvant therapy, with an overall five-year survival rate of 60%. The use of immunotherapy and targeted therapy improved treatment outcomes in patients with localized disease to over 70%. The paper presents a case of a 16-year-old patient who was admitted to the Department of Hematology, Oncology and Clinical Genetics of the Clinic for Pediatrics of the University Hospital Centre Rijeka due to several months of persistent pain in the distal part of the right thigh and swelling. The diagnosis of osteosarcoma was made by extensive diagnostic processing. Neoadjuvant chemotherapy started, and after three months, surgery - partial resection of the distal part of the right femur and reconstruction of the right knee with an endoprosthesis at the Clinic for Children's Diseases in Zagreb was made. Further treatment was continued at the University Hospital Centre Rijeka with adjuvant chemotherapy and intensive physical therapy. The measurements presented in this paper show the condition after surgery, and the effect of physical therapy in circumference and manual muscle test of the operated leg, the range of motion of the knee joint of the operated leg, and the longitudinal length of both legs during treatment at KBC Rijeka.