Direct measurement of maximum oxygen uptake (VO2max) is the standard index of cardiorespiratory fitness, but is practical only in a laboratory setting. Current cycle ergometer tests to estimate VO2max are difficult for inactive adults because most of these tests are lengthy and require a high initial exercise rate. We modified an existing test, the Astrand-Rhyming test, to avoid these problems. Maximum oxygen uptake was measured directly and estimated by means of our protocol in a test group of 50 men and women, ten for each decade between 20 and 70 yr, to develop multiple regression equations to correct for variations due to age. Equations for each sex were computed with directly measured VO2max as the dependent variable and with the estimated VO2max and age as independent variables. The validity of these equations was tested by deriving data from an additional 63 subjects (validity group). No significant differences were found between the directly measured VO2max and the VO2max estimated by our protocol and equations. For each group, the mean difference between the two values was less than 120 ml X min-1. Correlations between the measured and estimated VO2max ranged from 0.92-0.93 for the age groups. Our modification of the Astrand-Rhyming protocol accurately estimates VO2max and is safe and suitable for assessing cardiovascular fitness in epidemiologic studies of people between the ages of 20-70 yr.