A high incidence (40%) of cardiac arrhythmias was found in patients while on dialysis. This incidence was significantly higher than on nondialysis days. A comparison study of patients with significant cardiac arrhythmia and patients with cardiac arrhythmia was made. There was no difference in the echocardiogram, total body potassium, plasma renin activity, aldosterone, catecholamines, serum sodium, potassium and calcium between the two groups. Significant alkalinization occurred in all patients at the end of dialysis. Blood levels of total PTH and C peptide were higher in the arrhythmic patients versus the nonarrhythmic patients. No explanation was found as to why patients developed arrhythmias or what differentiated the two groups.