This study aimed at examining whether physiological exercise variables at the submaximal level, defined as 75% of the age-predicted target heart rate, are able to predict peak exercise performance in women with chronic fatigue syndrome (CFS) (n = 222). Subjects performed a bicycle ergometric test against a graded increase in workload until exhaustion with continuous monitoring of electrocardiographic and ventilatory variables. Oxygen uptake at the submaximal level (VO2SUBMAX) correlated strongly with peak oxygen uptake (VO2PEAK) (r = 0.7). For the prediction of VO2PEAK, linear regression analysis determined the line of best fit as:VO2(PEAK) = 0.95 X VO2SUBMAX + 372.3.Using this equation, the mean error in the prediction was 14.6 +/- 11.2% (range 0.1-63.7%). It is concluded that the prediction of VO2PEAK based on VO2SUBMAX might be useful for analyzing group differences or treatment effects but not for individual (clinical) purposes. (C) 2007 IMSS. Published by Elsevier Inc.