Snoring and obstructive sleep apnea (OSA) are related to narrowing of the upper airway. A mandibular advancement splint (MAS) could improve both conditions by increasing oropharyngeal and hypopharyngeal dimensions. The effects of a MAS on snoring and OSA was evaluated 3.5 +/- 2.1 (mean +/- SD) mo after issue in 57 subjects with habitual loud snoring, 39 of whom had an apnea-hypopnea index (AHI) > or = 10. Assessment was by questionnaire (all subjects) and polysomnography (51 subjects, 47 male) including measurement of sound intensity. Use of the MAS was randomized to first or second half of study. Snores were scored where inspiratory noise was greater than 5 dB above background. Total sleep time, sleep efficiency, % REM sleep, and % sleep spent supine were similar (p > 0.05) with and without the MAS. Snores per sleep minute, corrected for time in apnea, and sound intensity of snores (% snores > or = 50 dB) decreased with the MAS from 11.0 +/- 5.8 and 42.0 +/- 25.0% to 9.0 +/- 6.0 (p < 0.01) and 26.2 +/- 25.2% (p < 0.01), respectively. Using the MAS significantly improved OSA: AHI decreased from 32.2 +/- 28.5 to 17.5 +/- 22.7 (p < 0.01) and arousal index decreased from 31.4 +/- 20.6 to 19.0 +/- 14.6 (p < 0.01). AHI decreased to < 20 with the MAS in 12 of 17 subjects where untreated AHI was between 20 and 60, and in 2 of 9 subjects where untreated AHI was > 60. Forty-five patients continued to use the MAS regularly.(ABSTRACT TRUNCATED AT 250 WORDS)