We report two patients undergoing maintenance hemodialysis who presented with sleep apnea syndrome (SAS). The first patient is a 36-year-old man with a terminal Berger's glomerulopathy and associated obstructive sleep apnea syndrome (OSAS) (apnea-hypopnea index [AHI] = 80). He was receiving home hemodialysis and was treated by nasal continuous positive airway pressure (CPAP). After successful renal transplantation, his symptoms completely disappeared, and control polysomnography greatly improved (AHI = 9). The second patient had hypokalemic nephropathy with severe, uncontrolled hypertension and hypertensive myocardiopathy. He was receiving home dialysis and showed a central sleep apnea syndrome with an AHI of 51. He also was successfully treated by nasal CPAP. After renal transplantation, his sleep improved, insomnia disappeared, and polysomnography showed great improvement (AHI = 5). We discuss the role of periodic breathing related to end-stage renal disease associated metabolic abnormalities, as a pathogenetic factor of these SASs. Respiratory correction of chronic metabolic acidosis, "uremic toxins," "middle molecules," and hemodialysis are all evoked as etiologic factors and their own roles are discussed.