Haemodynamic abnormalities have been documented in the chronic fatigue syndrome (CFS), indicating functional disturbances of the autonomic nervous system responsible for cardiovascular control. This study was designed to explore the pathophysiology in adolescent CFS-patients by analysing RR-interval (RRI) variability and diastolic blood pressure (DBP) variability during mild orthostatic stress, using an algorithm which accounts for non-stationary biosignals. A total of 27 adolescents with CFS and 33 healthy control subjects having equal age- and sex distribution underwent 15 min of 20 degrees head-up tilt (HUT). The spectral power densities of RRI and DBP were computed in the low-frequency (LF) band (0.04-0.15 Hz) and the high-frequency (HF) band (0.15-0.4 Hz) using an adaptive autoregressive algorithm to obtain a time-varying spectrum. RMSSD, a time domain index of RRI variability, was also computed. At rest, all indices of variability were similar in the two groups. During tilt, CFS patients had a larger increase in the LF/HF ratio (P<or=0.001) and normalized LF power of RRI (P<or=0.01), and a larger decrease in normalized HF power (P<or=0.01) of RRI than controls. CFS patients also had trends towards a larger decrease in absolute HF power of RRI and a larger increase in normalized LF power of DBP. These findings suggest that adolescents with CFS have sympathetic predominance of cardiovascular regulation during very mild orthostatic stress. Possible underlying mechanisms are moderate hypovolemia, abnormalities of reflex control or physical de-conditioning.