PURPOSE OF THE STUDY The aim of this study was to evaluate and compare the functional outcomes and complication rates of patients in the midterm follow-up period when patients applied with medial unicondylar knee arthroplasty (UKA) were grouped according to Body Mass Index (BMI) values MATERIAL AND METHODS The study included 125 patients applied with medial UKA between December 2013 and December 2017 because of isolated medial compartment gonarthrosis and completed at least 2 years of follow-up. The patients were separated into three groups: non-obese with a BMI < 30 kg/m2, obese with BMI of 30-39 kg/m2, and morbidly obese (BMI ≥ 40 kg/m2). The implants used in all surgeries were cemented fixed-bearing implants. All patients were evaluated preoperatively with bilateral orthoroentgenograms, and weight-bearing anteroposterior and flexed lateral knee X-rays. Postoperatively, bilateral orthoroentgenograms and weight-bearing anteroposterior and flexed lateral knee X-rays were taken. Pre- and post-operative clinical outcomes were evaluated using the Knee Society Score (KSS), functional Knee Society Score (fKSS) Oxford Knee Score (OKS) and Visual Analogue Scale (VAS). RESULTS The 125 study patients comprised 83 females and 42 males with a mean age of 54 years (range, 42-71 years). No significant difference was determined between the three groups in respect of age, gender and side distribution (p > 0.05). In all three groups, the fKSS, KSS, OKS and VAS values at the final follow-up examination were statistically significantly improved compared to the preoperative values. No periprosthetic infection was determined in any patient throughout the follow-up period. Revision surgery was applied for various reasons to 5 patients; 2 in the non-obese group, 2 in the obese group, and 1 in the morbidly obese group. DISCUSSION In the last 20 years, a general increase has been seen in the number of obese patients with knee osteoarthritis requiring surgery. The increase in obesity represents an important worldwide public health problem. CONCLUSIONS The results of this study suggest that obesity does not influence the middle term outcomes of UKA. The functional outcome scores after surgery and improvement in these scores were comparable to patients with normal BMI. Key words: medial gonarthrosis, obesity, unicondylar knee arthroplasty.