Osteoarthritis (OA) is the most prevalent chronic joint disorder in the world. It is the leading cause of pain and disability among older people. The most frequently involved joints in OA are knees and hips. The disability and poor QoL associated with OA will continue to escalate as the average life expectancy increases. The study was conducted to determine the most important factors influencing QoL in knee/hip OA patients and to compare younger patients (40-65 years) with the elderly (66-80). A second goal was to study the effect of age on QoL. The WOMAC Osteoarthritis Index was used as an instrument to asses QoL. As a part of the study was performed a validation of the translated Polish version of WOMAC. WOMAC was administered twice between 7-10 days apart to 59 patients with knee/hip OA and then the results were compared with a global self assesment in VAS. High test-retest reliability, good validity and responsiveness of the Polish version of WOMAC was proved. In this study the age of OA patients seemed to be less important than the health status in determination of QoL. Regardless of age, in both groups the most important parameter of QoL evaluated by WOMAC was limitation in physical function. For the younger patients the most important factors which contribute to better QoL are the absence of pain-related sleep problems, good physical and mental functions with normal BMI. In the older group the most important factors are personal independent (high score in IADL), good physical functions measured by 25m walking test and scoring OA by Seyfried), good mental performance (AMTS), with absence of depression and normal BMI.