A high prevalence of sleep apnea syndrome has been reported in previous studies of patients with chronic renal failure. The possible effects of chronic hemodialysis on the magnitude and severity of sleep apnea have not yet been clarified. The present study was undertaken to understand this relationship, by examining subjective and objective measures of sleep on nights following hemodialysis compared to those without hemodialysis. Significant sleep apnea was noted in 6 of 11 patients. The percentage of apnea time comprised of obstructive apneas increased significantly on the nights following hemodialysis. No significant differences occurred between these nights in the subjective or EEG measures of sleep, or in the total number of disordered breathing events or level of arterial oxygen desaturation. The association between end-stage renal disease (ESRD) and sleep apnea syndrome remains highly significant, but seems not to be acutely altered by conventional hemodialysis treatment.