Osteoarthritis (OA) of the knees is a functionally limiting disability. Physical therapy (PT) is considered a useful treatment for OA, although evidence is qualitative. The purpose of this study was to quantitatively measure the effects of a 3-month PT program (n = 40; 20 men and 20 women) with knee OA. Muscle function and functional assessment parameters were measured. All data were analyzed by repeated measures analysis of variance (p < 0.05). There were no significant changes in handgrip strength and endurance, limb volume, or angular velocity after PT. Maximal muscle length was significantly increased. Muscle strength significantly increased for the hamstrings (9% and 19%) and quadriceps (8% and 24%) for the men and women, respectively. Endurance improved for the quadriceps (26% and 39%) and hamstrings (18% and 28%) for men and women, respectively. Functionally, there were significant improvements in the ability to climb stairs, rise from a chair, and walk. Walking time (50 ft) and the difficulty and pain of performing various activities decreased. Most improvements had occurred after 1 month of PT. For the first time, the effects of a PT program have been quantitatively measured for patients with knee OA.