Because tilt table testing can be influenced by a large number of personal factors, such as age, slight build, baseline BP, medications, exercise, familial tendencies, and diet, salt, and fluid intake,33- 36 the selection of control subjects is particularly critical. In previous work, controls were either not recruited12,28 or described as normal, healthy volunteers, without adequate reference to the nature or extent of their physical activity, salt intake, or diet.13- 15,26,27 In the current study, the healthy twin was, of course, identical in terms of age, sex, and genetics. Furthermore, even after the potential confounding influence of exercise capacity (O2 max) and salt and caffeine consumption was adjusted for, the lack of a difference in NMH between the twins with CFS and the healthy twins remained. The tilt table test was also completed under strictly blinded conditions during the same day for both twins. Such methodological considerations are crucial, since changes in diet owing to illness and cardiovascular deconditioning are possible explanations for the NMH observed in some subjects with CFS.