To determine if there was a relationship between cardiac arrhythmias and sleep apnea, we studied the prevalence of arrhythmias in a consecutive series of patients referred to our clinic for assessment of this disorder. Two hundred of 263 physician-referred patients were eligible for the study and, of these, 173 (86.5%) had complete investigations. All patients underwent a full night of polysomnography and Holter monitoring. Sleep apnea was diagnosed if patients had more than 10 apneas and hypopneas per hour (AHI). In 76 patients (43.9%) sleep apnea was diagnosed (median AHI = 33). The prevalence of arrhythmias in patients with sleep apnea versus those without was, respectively: complex ventricular ectopy (including ventricular tachycardia), 1.3% (95% CI, 0.4 to 6.9) versus 4.1% (CI, 1.6 to 10.1); frequent ventricular premature beats (> 30/h), 2.6% (CI, 0.8 to 8.9) versus 6.2% (CI, 2.9 to 12.8); second-degree atrioventricular block, 1.3% (CI, 0.4 to 6.9) versus 4.1% (CI, 1.6 to 10.1); sinus arrest, 5.2% (CI, 2.2 to 12.6) versus 1.0% (CI, 0.2 to 5.6). None of these differences was statistically significant. We conclude that the prevalence of cardiac arrhythmias is low in patients without serious cardiac or respiratory comorbidity who are referred for assessment of sleep apnea. Furthermore, the presence or absence of arrhythmias in this group is unrelated to sleep apnea severity.