The aim of this study was to investigate sleep-related sweating as a symptom of obstructive sleep apnoea (OSA). Fifteen otherwise healthy male non-smoking patients with untreated moderate-to-severe OSA underwent polysomnography, including measurements of skin and core body temperature and electrodermal activity (EDA) as an objective indicator of sweating. Evening and morning blood pressure was measured as well as catecholamines in nocturnal urine. All measurements were repeated after 3 months on successful continuous positive airway pressure (CPAP) treatment. The untreated OSA subjects had a mean (+/-SD) apnoea-hypopnoea index of 45.3 +/- 3.9 and a mean EDA index during sleep of 131.9 +/- 22.4 events per hour. Patients with higher EDA indices had higher systolic blood pressure in the evening and morning (P = 0.001 and 0.006) and lower rapid eye movement (REM) sleep percentage (P = 0.003). The EDA index decreased significantly to 78.5 +/- 17.7 in the patients on CPAP treatment (P = 0.04). The decrease correlated with lower evening systolic and diastolic blood pressure (P = 0.05 and 0.006) and an increase in REM% (P = 0.02). No relationship was observed between EDA and skin or core body temperature, or to catecholamine levels in urine. OSA patients who experience sleep-related sweating may have increased blood pressure and decreased REM sleep compared with other OSA patients. CPAP treatment appears to lower blood pressure and increase REM sleep to a higher extent in these patients compared with other OSA patients.