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The purpose of this study was to cross validate the equation developed by Rintala et al. (1992) to estimate the cardiorespiratory efficiency of men with mental retardation (MR). Subjects were 19 healthy men (27 ± 8 yr) with MR (IQ = 58 ± 12). Following familiarization, a graded maximal treadmill test and two 1-mile walk tests (Rockport Fitness Walking Test, RFWT) were administered. The peak ˙VO2 value was the criterion measure used to cross validate the equation. The equation was: Peak ˙VO2(ml·kg-1·min-1) = 101.92 - 2.356 (MILE) - 0.420(WEIGHT). The mean differences were 2.04 (MILE1)(P = 0.02) and 2.43 (MILE2)(P = 0.004) ml·kg-1·min-1. A significant positive correlation was found between measured peak ˙VO2 and predicted peak˙VO2 on both 1-mile walks (r = 0.91 and 0.93). For both predicted peak ˙VO2 values, the Total Error (TE) was greater than standard error of estimate (SEE), indicating a systematic difference between the measured and predicted peak ˙VO2 values. Moreover, only 58% of the measured peak ˙VO2 values fell within the prespecified range. Testretest reliability of RFWT was R = 0.96. However, the prediction equation underestimated the actual cardiorespiratory levels in 74% and 79% of the subjects, depending on the trial. Because the equation developed in this research underestimates the measured ˙VO2 values for the majority of these subjects, the test is probably not statistically valid, even if reliable, and the prediction formula needs to be revised for this population.