Part I of this two-part article presents a review of the symptoms, pathophysiology, and diagnosis of the sleep apnea syndrome. The more common obstructive type is characterized by disturbed sleep and daytime hypersomnolence. Subjective signs and symptoms should be correlated with objective findings by way of polysomnography (sleep study) to confirm the diagnosis and determine the severity of obstructive sleep apnea. Electrocardiographic monitoring during polysomnography has revealed potentially life-threatening arrhythmias during sleep. Differential diagnosis should include central sleep apnea and narcolepsy to avoid inappropriate therapy and worsening of symptoms.