The purpose of this study was to use an allometric model (maximal oxygen uptake (VO2,max) = FFMb1 × PAb2 × exp(b0 + b3 age + b4 sex) × ) to determine the influence of fat-free mass (FFM), physical activity (PA), sex and age on VO2,max in older men (n = 152) and women (n = 146) aged 55-86 years. VO2,max was measured during a fatigue-limited treadmill test, FFM was determined from skinfold thickness and physical activity by the Minnesota Leisure Time Physical Activity questionnaire. The model was linearised by taking the natural logarithm of VO2,max, FFM and physical activity. Variables were selected using multiple linear regression (P < 0.05). The sex variable was not significant (P = 0.062). The model explained 72.1 % of the variance in VO2,max. Significant individual coefficients were incorporated into the model yielding the following expression: VO2,max = FFM0.971 × PA0.026 × exp(-2.48 - 0.015age). Therefore, FFM and physical activity were significant factors contributing to the changes in VO2,max with age. In addition, controlling for FFM and physical activity abolished sex differences in VO2,max. The rate of decline in VO2,max (after accounting for FFM and physical activity) with age, was approximately 15 % per decade.