The consistency of apneas from night to night was examined in 2 groups of patients. The first group had more than 100 apneas per night (frequent apnea) and the second group had less than 100 apneas per night (infrequent apnea). All patients underwent clinical polysomnography for 2 nights, with no significant weight change or treatment occurring between recordings. The frequent apnea group showed a consistent number of apneas on the 2 nights (r = 0.92, p less than 0.01), whereas the infrequent apnea group showed a highly variable number of apneas (r = 0.35, p greater than 0.10). The correlations on apnea index (apneas per hours of sleep) showed a similar result. Apnea duration and type were consistent in both groups of patients.