We found that fitness and perception of exertion did not appear to mediate treatment effects, but that timed walking distance, assessed for the first time in our study, mediated the effect of GET. This suggests that increasing tolerance of physical activity might produce benefit without improving physical fitness. These findings are consistent with those of previous studies.8,9,32 A randomised controlled trial of GET for chronic fatigue syndrome 8,16 found that those who rated themselves as better were no fitter or stronger than the rest. A second trial of GET also suggested that physical reconditioning was not a mediator of the effect of treatment, but that a reduction in symptom focusing and increased exercise tolerance (as assessed by maximum heart rate achieved with exercise) mediated change in mental and physical fatigue.9 It must be acknowledged, however, that the walking test might not reflect activity or exercise levels in everyday life and might provide an explanation for why Wiborg and colleagues 33 found no evidence that actometer-measured physical activity mediated the effect of CBT.