An important methodological issue in measuring health-related quality of life is whether instruments such as SF-36 and EQ can be used on an elderly population. This paper aims to test the completion, reliability and validity of the SF-36 and Euroqol on an elderly female population, and to compare them with the OPCS Disability Survey. Three hundred and eighty women aged 75 and over participated in a randomized controlled trial of the use of clodronate provided the sample. As part of the trial, patients were asked to complete the UK SF-36 and Euroqol, and the OPCS disability survey instrument administered by interview in a hospital clinic at baseline. A random subsample of respondents were retested six months later. The SF-36 achieved poorer levels of completion by dimension (68.1%-88.9%) than the OPCS (99.2%) and Euroqol (84%-93.5%) instruments. There were no major floor effects in the distribution of scores, except for the role dimensions of SF-36. Correlation between test-retest were significant for all instruments, but lower for the role dimensions and social functioning of SF-36, and these dimensions also had 95% Cls for the mean differences in excess of 10 points. There was substantial agreement between the three instruments, and evidence for their construct validity against age and recent use of health services. The sensitivities of the instruments were tested through hypothetical changes in health status. There was some evidence of greater sensitivity to lower levels of morbidity in the SF-36. Where brevity is required and the health changes are expected to be substantial, then EQ may be sufficient. For greater sensitivity SF-36 seems to have an advantage, however lower completion rates and problems with consistency suggest it requires adaptation. One solution would be to use interviewer administration. Another would be to change the SF-36 to make it more suitable for use in elderly people, although this may reduce its usefulness as a generic instrument.