Auto-setting over-prescribed in six of 20 patients due to severe mouth leak. Mouth leak during CPAP has not previously been quantified, and these results, with leaks of 0.3-1.5 l/second, suggest a mechanism for the dry mouth and nasal symptoms commonly observed with CPAP. Obstructive events were reduced to the normal range in 19 of 20 patients and acceptably reduced in the 20th patient. In the nine nonleakers, mean CPAP pressure was reduced to 54% of the traditionally prescribed pressure. These preliminary results suggest that a self-setting CPAP machine, based on subtle indices of partial obstruction, is practicable in patients without severe mouth leaks.