Maximal exercise testing of mentally retarded adolescents and adults: reliability study. Few data are available regarding maximal exercise testing of mentally retarded individuals. No data are available on the reliability of maximal exercise testing of mentally retarded individuals. The purpose of this study was to determine the reliability of graded exercise testing of mentally retarded adolescents and adults. The testing was conducted at two geographically different centers. At Center A, 14 mentally retarded adolescents (11 boys, three girls) with Down syndrome, who were educable or trainable, were recruited from a nonresidential school. The subjects completed two Balke-Ware treadmill protocols until exhaustion. The treadmill time and heart rate (HR) were recorded. The time between tests was approximately one week. At Center B, 21 mentally retarded adults (14 women, seven men means IQ = 56) were recruited from local workshops and group homes. These subjects completed a treadmill walking protocol, with metabolic measurements, until exhaustion. The time between tests varied from one to four months. At Center A, the subjects achieved a mean treadmill time of 8.72min on test one and 8.84min on test two (means HR = 174 and 175bpm, respectively). The reliability coefficient between the two tests was .94. At Center B, the subjects achieved a mean V0(2)max of 27.2mL.kg-1.min-1 on test one and 26.9mL.kg-1.min-1 on test two. The reliability coefficient was .93. These data show that maximal exercise testing is reliable for these populations of mentally retarded individuals, exhibiting similar values to their nonretarded peers. 