The relationship between upper arm dimensions and maximal oxygen uptake (VO2peak) during arm exercise was investigated. Nineteen almost untrained male subjects (22 +/- 2 yrs, 183 +/- 5.5 cm, 74 +/- 7.5 kg; mean +/- SD) volunteered to take part in the study. The following anthropometrical upper arm data were obtained for each subject: upper arm circumference, biceps and triceps skinfold thickness, upper arm length. From these data the cross-sectional area of the muscles, of the muscles plus bone of the upper arm, and the upper arm muscle volume were calculated. Upper arm volume was determined by water immersion. The subjects performed arm cranking with an increasing load of 10 watt each minute (Wpeak 125 +/- 17.4 watt, VO2peak: 2.54 +/- 0.33 l/min; mean +/- SD). Multiple regression analyses showed that VO2peak could be best explained by the cross-sectional area of the muscles plus bone of the upper arm (r = 0.65, p < 0.005). Other factors did not add significantly. The variables as body mass, fatfree body mass, upper arm volume and total arm volume correlated less well with VO2peak (r < 0.5). In conclusion it is suggested to use the easily obtainable cross-sectional area of the muscles plus bone of the upper arm to standardize VO2peak of arm cranking.