Serial 12-lead electrocardiogram (ECG) recordings and 24-hour Holter monitoring were carried out in 39 patients with chronic renal failure, starting just before a routine dialysis session. In an attempt to identify risk factors for cardiac arrhythmias, the results obtained from each patient were correlated with a variety of clinical, hematological, and biochemical data. All patients exhibited ECG changes, which were most pronounced during the first 2 hours of dialysis. The most frequent of these changes were a decrease in T wave amplitude and increase in Tmax time (all patients), an increase of QRS amplitude (61% of patients), shortened or prolonged QTc interval (61%) and ischemic-like ST-T changes (22% and 39%, respectively). Potentially clinically significant arrhythmias occurred in 12 patients (31%) of which 8 were supraventricular, 3 were combined ventricular and supraventricular, and 1 was pure ventricular. The only clinically identified risk factor for complex ventricular and supraventricular arrhythmia was advanced age. The arrhythmia and nonarrhythmia groups differed significantly in their predialysis hematocrit, O2 content, serum urea, and osmolarity, and in their postdialysis serum phosphorus and osmolarity. The results indicate that patients with chronic renal failure frequently exhibit ECG changes and a high incidence of ventricular and supraventricular arrhythmias, which may be prognostically significant, during and after hemodialysis.