Sleep apnea patients were studied three times prior to and 4 wk after a trial of nasal continuous positive airway pressure to determine the measurement properties of the Calgary Sleep Apnea Quality of Life Index (SAQLI), a disease-specific quality of life questionnaire. All patients completed the Medical Outcome Survey Short Form (SF-36), the Ferrans and Powers Quality of Life Index, and a global assessment of quality of life before and after treatment. The SAQLI was found to have a very high responsiveness index of 1.9 and an effect size of 1.1, which was much greater than the domains of the SF-36 and the FPQLI. There were statistically significant longitudinal correlations (range: 0.24 to 0.54) between the SAQLI and seven of the SF-36 domains in a pattern that was predicted beforehand and which demonstrated the validity of the SAQLI as an evaluative instrument. The SAQLI also had a range of correlations at baseline with the SF-36 (range: 0.36 to 0.71), the Epworth Sleepiness Scale (-0.26), and the FPQLI (0.29 to 0.66), and with a global rating of quality of life (0.61). The SAQLI had a high reliability coefficient of 0.92 on testing and retesting at 2 wk. We conclude that the SAQLI has excellent measurement properties for an evaluative instrument, and beginning evidence of validity as a discriminative index. It measures components of quality of life that are important to sleep apnea patients, and will be an important outcome measure in clinical trials.