Osteoarthritis (OA) is a common chronic and progressive degenerative joint condition. A major consequence of knee OA is physical disability; especially difficulty with activities requiring ambulation and transfer, which are necessary to maintain independence and a good quality of life. The purposes of this study were to determine the efficacy of a muscle exercise program along with education about knee care, and investigate the clinical factors which influence its therapeutic efficacy. A prospective study was carried out among elderly Thai people aged 60 years and over, living in an urban community of the Bangkok metropolitan area around Siriraj Hospital, between October 1997 and September 1999. The number of elderly people with osteoarthritis of the knee was 392 (male:female=86:306) with a mean age of 67.7+/-6 years. They were interviewed for demographic data and information about their symptoms. The range of the knee movement, quadriceps strength and the six-minute walking distance and a knee X-ray were evaluated. Group exercise was arranged twice a week for 8 weeks for the intervention group only. The results of the study revealed that the quadriceps strength in the intervention group had a tendency to increase and the walking distance was significantly improved especially in the first two months but there was a tendency to decline by the sixth and twelfth month after enrollment. On the other hand, there was no statistically significant difference of both important outcomes for the control group when assessed in the second month. The findings at the sixth and twelfth month after enrollment were that the quadriceps strength was slightly increased but the walking distance was decreased when compared with the initial assessment. An exercise frequency of 12 sessions in two months was sufficient to improve muscle strength and walking distance. Group exercise produces a significant improvement in strength and walking ability, especially in the first two months. Deterioration over time could be due to multiple factors, such as lack of regular exercise, lack of motivation, lack of family support or poor economic status, so we should encourage the elderly to exercise regularly.