105 patients with Kellgren-Lawrence grade III knee osteoarthritis were randomized to treatment with either three weekly intraarticular hyaluronic acid injections or a 6-week exercise therapy program. Patients were assessed for improvement in pain variables, knee range of motion, and the Hospital for Special Surgery (HSS) score up to 18-month follow-up. While pain at rest and pain during stair climbing did not significantly differ between groups, pain during activity favoured IAHA at 6 weeks and 3 months, while pain during transfer activities favoured exercise therapy from 1 week to 6 weeks. HSS score favoured IAHA at 3 months, but favoured exercise therapy at 12 months. For all outcomes, 18-month differences between groups were not statistically significant.