We evaluated the possibility that in some patients with obstructive sleep apnea, the initial polysomnogram may be negative. We reviewed polysomnograms performed at the Medical College of Georgia from 1984 to 1990 and found nine patients whose initial polysomnogram was negative but whose repeat polysomnogram confirmed obstructive sleep apnea. All nine patients (five women and four men; average age, 44.2 years) had an apnea index of less than 5 (fewer than five apneic episodes per hour) and had a total of fewer than 20 apneic episodes during the initial overnight polysomnogram. The change in average weight was not significant. Three patients had received short-term oxygen therapy, and two of these three received nasal continuous positive airway pressure prior to the initial study. The time that patients spent supine increased from 101 min in the initial study to 180 min in the second, but this was not significant (p = 0.12). Comparison of the initial and diagnostic polysomnograms showed significantly reduced total sleep time (from 3.75 +/- 1.84 h to 5.32 +/- 1.11 h; p = 0.04) and reduced rapid eye movement (REM) sleep time (from 0.27 +/- 0.27 h to 0.75 +/- 0.58 h; p = 0.037) in the initial study. We conclude that in a small subset of patients with obstructive sleep apnea, the initial polysomnogram may be falsely negative, which could be due to previous therapy, a reduction in total sleep time and REM sleep, or other unidentified factors.