Exercise testing is often performed in persons with cardiac disease to measure their functional capacity. Physical activity questionnaires assessing functional capacity have been used as a low-cost and convenient alternative to exercise testing, but have not been well validated against measured oxygen consumption in a cardiac population. This study assesses the ability of a simple, 13-item, self-administered activity questionnaire, known as the Specific Activity Questionnaire (SAQ), to measure functional capacity prospectively in a large sample of cardiac patients. Ninety-seven consecutive cardiac out-patients (85 men and 12 women aged 59 +/- 10 years [mean +/- SD]) completed the SAQ before an elective symptom-limited treadmill test. Subjects returned within 10 days to repeat the treadmill test, following the same protocol, with the additional measurement of peak oxygen consumption, VO 2 (ml . kg - 1 .min - 1 ), using open circuit spirometry. The SAQ score was significantly related to measured peak VO 2 (r = 0.57, p < 0.001). Stepwise multiple linear regression analysis found that the addition of patient age, height, and body weight to SAQ score improved the measurement of peak VO 2 , accounting for 51% of the sample variance (R = 0.71, p < 0.001). Peak VO 2 was obtained from the following regression formula: VO 2 = (2.36)SAQ + (0.35)HEIGHT - (0.19)AGE - (0.16) BODY WEIGHT - 33.89; SEE 5.43. Thus SAQ, a simple 13-item self-administered activity questionnaire, is able to provide a moderately good measure of functional capacity in cardiac patients and may be a useful tool in studies of the cardiac population when formal exercise testing is impractical or uneconomical.