Increasing recognition of sleep-disordered breathing (SDB) and its morbidity have prompted reevaluation of techniques to identify respiratory events during sleep. The present study was designed to evaluate the utility of various metrics of SDB and to identify the optimal respiratory metric that objectively correlates to symptoms of excessive daytime somnolence (EDS). Metrics were derived from combinations of conventional apnea/hypopnea, flow limitation events (transient elevated upper airway resistance identified by characteristic flattening on the flow/time tracing, using a noninvasive nasal cannula technique), desaturation, and arousal. A total of 137 subjects underwent clinical evaluation and nocturnal polysomnogram. In 34 randomly selected subjects, the best metrics for discriminating between 13 subjects with no EDS/snoring and 21 patients with EDS and snoring were identified by receiver operator curve analysis. Of the metrics and cut points tested, a total respiratory disturbance index (RDI(Total), sum of apneas, hypopnea, and flow limitation events) of 18 events/h was found to have the best discriminant ability (100% sensitivity and 96% specificity). Prospective testing of this metric was then performed with the remaining 103 subjects (14 nonsnoring non-EDS, 21 snoring non-EDS, 68 snoring with EDS). Using this cutoff of 18 events/h, we obtained 71% sensitivity and 60% specificity for identifying subjects with EDS. We conclude that, in subjects with upper airway dysfunction, an index that incorporates all respiratory events provides the best quantitative physiological correlate to EDS.