We examined the predictive value of history and physical examination in the diagnosis of obstructive sleep apnea (OSA) syndrome. This was achieved by studying a set of 594 patients referred to the sleep clinic because of suspicion of sleep apnea. All patients were asked a set of standard sleep-related questions and all had nocturnal polysomnography. We used stepwise multiple linear regression analysis to examine the relationship between the apnea/hypopnea index (AHI), defined as the number of episodes of cessation of breathing per hour of sleep (dependent variable), and age, sex, body mass index (BMI) and replies to the sleep questionnaire (independent variables). We found that age, sex, body mass index, bed partner observation of apnea and pharyngeal examination were significant predictors of AHI, explaining 36% of the variability. Subjective impression of the examining clinician was also an independent significant predictor of AHI, accounting for 10% of the variability. Using a conventional cutoff value of 10 to divide patients into apneics (AHI > 10) and nonapneics (AHI < or = 10), the sensitivity of subjective impression was 60% and the specificity 63%. We conclude that although clinical features obtained during history and physical examination explain a relatively high percent of the variability in AHI, subjective clinical impression alone is not sufficient to reliably identify patients with or without sleep apnea.