Hot bathing has been associated with sudden death and so the present study investigated its effects on autonomic activity and hemodynamics in the elderly patient and the healthy young by analyzing heart rate variability (HRV). Subjects were 9 elderly men (mean age, 75 years) and 9 young men (mean age, 27 years), who were immersed up to shoulder level while in a sitting position for 10min with the bath temperature at 40 degrees C. Blood pressure (BP) and heart rate (HR) were monitored. BP in the young decreased during bathing (p<0.01), whereas in the elderly BP had a maximum value just at the start of immersion (p<0.05) with a slight decline at 4 min after the start of immersion. Although HR in the young increased (p<0.01), in the elderly there was an abrupt increase in HR just at the start of immersion (p<0.05), followed by a decrease in HR. With regard to HRV, the high-frequency (HF) component in the young men was suppressed during immersion (p<0.01), but was unaffected in the elderly. The LF (low frequency)/HF ratio in the elderly decreased at 4 min (p<0.05). In conclusion, hypotensive syncope may cause sudden death by drowning during hot bathing, and is a consequence of the decrease in sympathetic tone that develops approximately 4min after immersion.