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Data on moderately cold water immersion and occurrence of arrhythmias in chronic heart failure (CHF) patients are scarce.We examined 22 male patients, 12 with CHF [mean age 59 years, ejection fraction (EF) 32%, NYHA class II] and 10 patients with stable coronary artery disease (CAD) without CHF (mean age 65 years, EF 52%). Haemodynamic effects of water immersion and swimming in warm (32°C) and moderately cold (22°C) water were measured using an inert gas rebreathing method. The occurrence of arrhythmias during water activities was compared with those measured during a 24 h ECG recording. Rate pressure product during water immersion up to the chest was significantly higher in moderately cold (P=0.043 in CHF, P=0.028 in CAD patients) compared with warm water, but not during swimming. Rate pressure product reached 14200 in CAD and 12 400 in CHF patients during swimming. Changes in cardiac index (increase by 5–15%) and oxygen consumption (increase up to 20%) were of similar magnitude in moderately cold and warm water. Premature ventricular contractions (PVCs) increased significantly in moderately cold water from 15 ± 41 to 76 ± 163 beats per 30 min in CHF (P=0.013) but not in CAD patients (20 ± 33 vs. 42 ± 125 beats per 30 min, P=0.480). No ventricular tachycardia was noted.Patients with compensated CHF tolerate water immersion and swimming in moderately cold water well. However, the increase in PVCs raises concerns about the potential danger of high-grade ventricular arrhythmias.