Repeatability of sites of obstructive events is influenced by the severity of illness and the degree of upper or lower obstructive predominance. The distribution of sites of obstructive events (classified as mainly "upper"/"lower" or as percent upper obstructive events of all) can be identified with relative confidence in patients who have frequent apneic events (AI > or = 5) or a high degree of upper or lower obstructive predominance and especially in those who have a combination of these two criteria.