We investigated the frequency of cardiac arrhythmias in patients suspected of having sleep apnea, and related them to the severity of apnea, snoring, and nocturnal hypoxemia. We prospectively studied 458 patients who had nocturnal polysomnography which included objective measurement of snoring (quantified by the number of snores per hour of sleep [snoring index (SI)] and maximum nocturnal sound intensity [(dBmax)], as well as examination of the electrocardiogram (modified lead 2). We found 58 percent prevalence of arrhythmias in patients with sleep apnea (apnea/hypopnea index = AHI > 10), vs 42 percent in nonapneic controls (chi 2 = 16.7, p < 0.0001). Patients with arrhythmias had more severe apnea and nocturnal hypoxemia, but not snoring, than patients without arrhythmias. To examine separately the relationship between the prevalence of arrhythmias and snoring, nocturnal oxygenation, and apnea--we selected subgroups of patients "at the opposite ends of the spectrum" with respect to the severity of snoring, hypoxemia, and apnea. We found that 38 percent of light snorers had arrhythmias vs 39 percent of heavy snorers, 82 percent of patients with mean nocturnal oxygen saturation < 90 percent had arrhythmias vs 40 percent of patients with mean nocturnal oxygen saturation > 90 percent (chi 2 = 7.4, p = 0.006), and 70 percent of patients with AHI > or = 40 had arrhythmias vs 42 percent with AHI < or = (chi 2 = 9.2, p = 0.002). We conclude that patients with sleep apnea as a group have higher prevalence of cardiac arrhythmias than nonapneic patients and that snoring alone, without concomitant sleep apnea, is not associated with increased frequency of cardiac arrhythmias.