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BACKGROUND In some stroke patients blood pressure (BP) fluctuates extensively during medical rehabilitation, so the present study investigated the influence of autonomic nervous dysfunction on the change in BP during exercise. METHODS AND RESULTS The subjects consisted of 55 stroke inpatients (males, 29; mean age, 58.8 years old; ischemic/hemorrhagic etiology, 30/25) who were admitted to the Stroke Center within 2 weeks of their first stroke. The control group consisted of 15 age-matched healthy volunteers. The 24-h heart rate (HR) variability (HRV) and BP variability (BPV) were examined, and then the increase and recovery of BP and HR were measured during bicycle ergometer exercise at 4 METs. Components of 24-h HRV (low-frequency power (LF), high-frequency power (HF), LF/HF, and asleep-awake ratio of LF/HF (LF/HF(d-n)) were lower (p < 0.01) and BPV was greater in the stroke group (p < 0.05) than in the control group. There was a negative correlation between BP change during exercise and LF/HF or LF/HF(d-n) (r = -0.43 or r = -0.58, p < 0.01), and a greater increase in systolic BP (102 +/-9.8 mmHg, n = 7) during exercise was observed in stroke patients with lower LF/HF(d-n) (< or = 1.0). CONCLUSIONS Lower HRV in stroke patients may relate to an increase in BP during exercise. HRV is useful for estimating the risk during medical rehabilitation.