An epidemiological study of 392 elderly Thai patients with osteoarthritis (OA) of the knee, 86 males and 306 females, with a mean age of 67.8 years, was performed. The prevalence of knee OA was 34.5-45.6 per cent. Most of the patients had completed primary education and had sufficient income. More than half of the patients had pain in both knees, but many of the elderly people used neither knee supports nor walking aids (82.7 and 90.6%). Sixty-four per cent had no previous knowledge concerning the value of exercise. Sixty-five per cent of the patients had never used any pain medication. Three-fourths of the subjects evaluated themselves to have fair to good health. Sixty per cent presented with knee pain when climbing the stairs, and 64.8 per cent had a fair quality of life. Almost all the subjects had mild to moderately severe grade of OA and their X-ray findings were level II-III. The average quadriceps strength and six-minute walking distance were 12.95 +/- 5.51 kilograms and 363.69 +/- 99.34 meters respectively. The five factors that significantly correlated with walking ability were age, sex, the functional incapacity score, using walking aids and level of severity found on X-ray. Previous knowledge about knee exercise did not affect walking ability, but may have been due to the irregularity of the exercise schedule they performed. The elderly should be encouraged to exercise regularly in addition to receive knowledge about proper exercise.