Ninety-eight elderly patients with knee osteoarthritis (OA) were randomized to receive an intra-articular injection of ozone (n=63) or placebo (n=35). Outcomes of interest included pain scores on a Visual Analogue Scale (VAS), the Lequesne index, the Geriatric Pain Measure (GPM) scale, the Western Ontario and McMaster Universities (WOMAC) Osteoarthritis Index scores for pain, stiffness, and function, the Short-Form 36 (SF-36) quality of life measure for functional capacity, physical health, general health, social aspects, and emotional aspects, and the incidence of adverse events. Outcomes were assessed at 4, 8, and 16 weeks post-treatment. Results of this study revealed that all outcomes were statistically significantly in favour of the ozone group at all time-points, with the exception of WOMAC stiffness scores which were in favour of the ozone group at 4 and 8 weeks only, and VAS pain scores were in favour of the ozone group at 8 and 16 weeks only (p<0.05 for all). Finally, 2 cases of adverse events in the placebo group and 1 case of adverse event in the ozone group were reported.