127 patients on ambulatory in-hospital haemodialysis (HD) were randomly selected from 13 centres. The prevalence of ischaemic heart disease was 14%, 17% had left ventricular dysfunction, and 37% had left ventricular hypertrophy. Ventricular arrhythmias, assessed by continuous 48 h Holter monitoring, were present in 76% of patients. 29% had greater than or equal to 2 premature ventricular complexes (PVC)/h; 21% had Lown classes 4A or B; and 6% had greater than or equal to 3 consecutive beats of ventricular tachycardia. Risk factors for ventricular arrhythmias were age greater than or equal to 55 yr and left ventricular dysfunction, independent of age. The frequency of ventricular arrhythmias rose significantly during the third hour of HD, and this effect lasted for at least 5 h after dialysis, and was associated with age greater than or equal to 55 yr, left ventricular dysfunction, and the presence of ventricular arrhythmias before dialysis. Ventricular arrhythmias are present in a large proportion of patients with end-stage renal failure and HD has an arrhythmogenic effect.