There are no studies on oxygen uptake of groups of physically active subjects aged over 70. This study describes the maximal oxygen uptake (VO2max) of 153 elderly people who completed the Nijmegen annual 4-day march (at least 30 km.day-1) in 1993. A total of 97 men with a mean age of 76.7 (SD 4.6) and 56 women with a mean age of 72.8 (SD 3.6) years participated in the study. The VO2max was determined using incremental cycle ergometry; 91 men and 49 women completed a maximal exercise test. Criteria for maximal performance were respiratory exchange ratio equal to or greater than 1.00, vertilatory equivalent for oxygen equal to or greater than 30.00 and maximal heart rate equal to or greater than (beats.min-1) 210 minus age (years). Mean maximal power output was 148.2 (SD 27.2) W and 120.4 (SD 20.5) W, mean VO2max.body mass-1 was 26.8 (SD 4.9) ml.kg-1.min-1 and 24.6 (SD 4.7) ml.kg-1.min-1, mean maximal heart rate was 152 (SD 18), and 157 (SD 14) beats.min-1 in men and women respectively. The mean VO2max.body mass-1 was about 20% higher than reported in other studies on subjects over 70 years of age. Mean maximal heart rate was about 10 beats.min-1 higher than predicted from the equation 220-age. The negative effect of chronic disease on VO2max.body mass-1 was smaller than in a sedentary reference population. The mean decline in VO2max.body mass-1 with age was 0.46 and 0.38 ml.kg-1.min-1 per year in the men and women respectively, which is the same rate as found in younger subjects. It was concluded that regular exercise might substantially increase aerobic power in the physically active elderly, even when they have chronic disease, and that it is unlikely that there is an accelerated loss of aerobic power in physically active elderly people aged over 70 year.