Uvulopalatopharyngoplasty is the treatment of choice for selected patients with obstructive sleep apnea, although the response to surgery is variable. We measured, in addition to obstructive apnea, the frequency of sleep-related partial upper airway obstruction in 11 patients with the obstructive sleep apnea syndrome both before and after uvulopalatopharyngoplasty. Partial obstruction was detected indirectly by recording the secondary hemodynamic changes and respiratory stimulation with the static charge sensitive bed. The frequency of obstructive apnea episodes was reduced from 56.3% to 18.4% by the operation, while the episodes of increased respiratory resistance rose from 3.6% to 20.4%. The various breathing anomalies decreased significantly, although they were still present during 75.6% of the recording time. The results suggest that uvulopalatopharyngoplasty substantially reduces the frequency of obstructive apnea, some of which persists as partial obstruction. The persistent partial obstruction may be the key factor contributing to further pharyngeal narrowing and a recurrence of obstructive sleep apnea syndrome.