Ever since Sullivan introduced nighttime nasal continuous pressure on the upper airway (CPAP) in 1981 it has been the standard treatment for sleep-hypopnea syndrome (SAHS). However, CPAP is carried out at great expense and is not tolerated by all patients. Moreover, its efficacy is dependent on the degree of compliance. In this study we set out to analyze the degree of compliance with CPAP over the first 3 months of treatment in a group of 142 consecutive patients with moderate to severe SAHS (apnea-hypopnea index: 48.9 +/- 20). Diagnosis and measurement of the level of CPAP needed (9.6 +/- 2.5 cm H2O) were based on polysomnography. Eighteen (13%) patients did not return for follow-up evaluation. In the remaining 124 patients (age 54 +/- 11 years) compliance with treatment was evaluated by way of a sleep diary in which the patient recorded the hours CPAP was used at night; this record was compared with readings from the CPAP generator's counter. All subjects were asked about their degree of sleepiness before treatment by way of a standard questionnaire. Although most patients reported regular use of CPAP in diaries, only about 60% actually used it for longer than a mean 4.5 hours daily. The most compliant patients could not be differentiated from the least compliant with respect to degree of initial sleepiness, apnea-hypopnea-per-hour index or level of CPAP required.(ABSTRACT TRUNCATED AT 250 WORDS)