Overnight polysomnography (ONP) is the "gold standard" for the diagnosis of sleep-disordered breathing, but it is expensive and time-consuming. Thus, daytime nap studies have been used as screening tests. If the findings of a nap study are normal or mildly abnormal, should ONP be performed? Do specific abnormalities in nap studies predict abnormal findings in ONP? To answer these questions, we conducted this study.