Twenty-seven morbidly obese patients (13 men and 14 women) with body mass index greater than or equal to 40 kg m-2 were examined. The mean age of the subjects was 36.9 +/- 8.2 years (range 23-51 years), and the mean BMI was 50.2 +/- 6.2 kg m-2 (range 40.0-62.9 kg m-2). A whole-night sleep recording was made for all patients with signs or symptoms indicative of possible obstructive sleep apnoea syndrome (OSAS). If the first nocturnal sleep recording was abnormal, it was controlled after 1 year. Eleven (10 men and one woman) of the 27 patients had an oxygen desaturation index (ODI) of 10 h-1. They were symptomatic with excessive daytime sleepiness or other daytime symptoms of OSAS. The occurrence of OSAS in men and women was 76.9 and 7.1%, respectively. Arterial hypertension was associated with OSAS, but not with smoking or the degree of obesity. Antihypertensive treatment was received by nine of the 27 patients; six of them had OSAS. Thus six of the 11 (54.5%) patients with OSAS and three of the 16 (18.8%) nonapnoeic patients were treated for arterial hypertension (Fisher exact test, P = 0.042). The odds ratio of OSAS for arterial hypertension is 5.2 (95% CI, 0.71-43.6). Vertical-banded gastroplasty was performed in 14 patients, three of whom had OSAS. The selection of patients for gastroplasty was made without taking into account the results of sleep recordings. In the three OSAS patients, a 30-38% reduction in BMI was achieved by surgery. Eight patients with OSAS were treated with an intensified dietary regimen, and the reduction in BMI ranged from -2.6 to 33%. OSAS was either cured or significantly improved in six (55%) patients, with a mean reduction in BMI of 27%, while in patients with persistent OSAS the mean reduction in BMI was only 7%.