PURPOSE: To assess the relationship between objectively-measured physical activity intensity (PAi) and VO2max in 275 adult females in a cross-sectional study controlling for the effects of age, percent body fat (%BF) and physical activity volume (PAv). METHODS: All subjects were female, non-smokers with a mean age of 40.1+3.0 years. PA was measured using accelerometers worn at the hip for 7 consecutive days. PA counts were collapsed into 10-minute epochs. PAv was the sum of all the PA counts over the 1008 epochs. Counts from at least 9 (>90 minutes) of the most intense epochs within specific ranges (<30,000 = low, 30,000–49,999 = moderate, and >50,000 = high intensity) were the basis for the PAi categories. A graded maximal treadmill test was used to assess VO2max. A mass spectrometer, medical gas analyzer measured the expired gases. Percent body fat was estimated using the Bod Pod®. Regression analysis was used to assess the relationship between PAi and VO2max. Partial correlation was employed to determine the effects of potential confounding variables on this relationship. RESULTS: Mean 7-day PAv was 2.7 +0.8 (+SD) million counts. Mean VO2max was 34.2 + 6.6 ml/kg/min and mean %BF was 31.8% + 6.9. The low (N=145), moderate (N=70), and high (N=58) PAi groups differed significantly in VO2max with means (ml/kg/min) of 31.9 + 5.2, 34.1 + 5.3, and 40.7 + 7.3, respectively (R2=0.26, F=48.5, p<0.0001). After adjusting for age, the significant differences remained (R2=0.26, F=48.4, p<0.0001). After controlling for PAv, mean differences across the PAi groups were significant (R2=0.15, F=27.4, p<0.0001). After controlling for %BF, differences in VO2max across the PAi groups remained significant (R2=0.10, F=33.1, p<0.0001). After controlling for age, PAv, and %BF simultaneously, VO2max differences across the PAi groups were significant (R2=0.055, F=17.7, p<0.0001). The full model, including the 3 potential confounding variables plus PAi, accounted for 58.4% of the variance in VO2max (R2=0.58, F=53.5, p<0.0001). CONCLUSIONS: PAi explained a significant portion of the variance of VO2max, even after controlling for age, PAv, and %BF. Apparently, PAi is an important factor accounting for differences in fitness in middle-age women. Most of the variance in VO2max can be explained by age, %BF, PAv, and PAi in combination.