Adverse cardiac and pulmonary events are frequently observed during hemodialysis and contribute to significant morbidity and mortality. The temporal relationship between these events during the intradialytic period has not been well defined. To examine the event rate and timing of silent ischemia, cardiac ectopy, and hypoxemia, we conducted a prospective, single-blind, randomized study of 10 subjects undergoing maintenance hemodialysis with four contiguous combinations of dialysis membranes (cuprammonium or polysulfone) and dialysates (acetate or bicarbonate). The frequency of oxygen desaturation events peaked during the first 2 hours, whereas silent myocardial ischemia and supraventricular ectopies occurred more often in the later hours. Ventricular ectopy occurred steadily throughout the intradialytic period. The combination of acetate dialysis and cuprammonium membrane is associated with the most frequent events. We conclude that cardiopulmonary events can occur frequently during hemodialysis, and the frequency is dependent on the type of dialysis membrane and dialysate buffer used.