Objectives: The objectives of this study are to examine the success of stem cell application treatment, by monitoring recovery using the WOMAC index, in patients with osteoarthritis of the knee. It also aims to examine the occurrence of side effects and adverse effects after the procedure in relation to the functional findings in the form of swelling, stiffness or inflammation of the knee or surface abdomen. Study Design: This study was designed as a cross-sectional study. Patients and Methods: The study involved 17 patients (6 women and 11 men). Subjects were treated with a stem cell application for osteoarthritis of the knee at the Department of Orthopedics, University Hospital Center Osijek. General data (age, sex, grade of arthrosis, and localization of the change) were collected from the available medical documentation. The functional findings of the affected joint were determined before and three months after the procedure using the WOMAC index. The occurrence of possible adverse effects and side effects was also examined postoperatively. Results: After treatment of osteoarthritis of the knee with the stem cell application, there is an improvement in all domains of the WOMAC index (Wilcoxon's test of the sum of wounds for individual domains is: the median of pain domain was 9 preoperatively, and the median on control examination was 3; the median of stiffness domain was 2 preoperatively and on control examination was 0; the median of function domain was 24 preoperatively, and the median on the control examination was 6) and in the total WOMAC index (Wilcoxon sum test, the median of the whole quality of life scale was 35 preoperatively, and on the control examination median was 9 (higher number means the poorer quality of life)). The most common side effect after surgery was swelling of the knee. Knee pain and stiffness, as well as redness, bleeding and pain on the stomach surface were reported. All side effects and adverse effects disappeared within a month of surgery. Conclusion: The use of adipose stem cells in the treatment of knee osteoarthritis is safe and effective and shows potential in reducing pain, and stiffness and improving joint function.