The relationship between orthostatic tolerance and aerobic fitness has yet to be accurately defined. Positive, negative and an absence of a relationship have all been reported in the scientific literature. This study determined tolerance to Lower Body Negative Pressure (LBNP, a measure of orthostatic tolerance), maximum oxygen uptake and blood volume in subjects in the trained and detrained state and in control subjects to examine whether a loss of aerobic fitness alters orthostatic tolerance. Measurements were made on seven athletic subjects (age 27.9 ± 3.3 years) before and after 17 weeks of training or detraining. A control group of seven normal fit subjects (age 28.7 ± 3.9 years) had the same measures taken before and after a control period of a similar duration. Data are given as means ± S.D. Between and within group differences were examined using Wilcoxon comparisons. For the test group the detrained state was associated with significantly lower (p < 0.01) aerobic fitness (maximum oxygen uptake reduced from 63.5 ± 9.9 to 53.6 ± 5.9 ml kg^-1min^-1) and blood volume (5.7 ± 1.0 to 5.3 ± 0.9 l) than the trained state. A significant increase (p < 0.01) in tolerance to LBNP as described by the cumulative stress index (product of duration and negative pressure endured before pre-syncope) was also found (increased from 785.7 ± 206.7 to 966.1 ± 337.9 mmHg min^-1). LBNP tolerance in the detrained state proved to be significantly greater (p < 0.05) than that of the control group (747 ± 185.7 and 738 ± 195.0 mmHg min^-1 pre and post control period respectively). No changes in these variables occurred in the control group (p > 0.05) (maximum oxygen uptake 45.5 ± 8.7 and 44.3 ± 8.9 ml kg^-1min^-1 and blood volume 5.28 ± 0.8 and 5.27 ± 0.8 l). Previous studies attempting to ascertain the relationship between aerobic fitness and orthostatic tolerance have rarely included control groups. The absence of a control group in the present study would have lead to the conclusion that the highly fit (trained) state is associated with low tolerance to orthostatic stress, and that detraining returns tolerance towards greater, normal fit values, a finding often reported in the open literature. The interesting finding that there was no difference between normal fit (control) and high fit LBNP tolerance and that a reduction of maximum oxygen uptake was in fact associated with tolerance to LBNP above that of normal fit subjects, supports the hypothesis that there could be a parabolic relationship between orthostatic tolerance and aerobic fitness under these specific circumstances.