The purpose of this thesis was to demonstrate in a randomised controlled trial whether BP was higher in untreated OSAHS. The study was crossover in design with the patients acting as their own controls, so increasing the power of the study. Sixty-eight patients were studied with at least two major symptoms of OSAHS, and a mean AHI of 15 events/hour slept and a mean age of 49 years (27-77). Diastolic Blood Pressure (DBP) was reduced by CPAP in patients with OSAHS. These data were analysed on an intention to treat basis, including all 68 patients including poorly compliant patients. This showed a 1.5 mmHg drop in DBP (p = 0.043) with CPAP. In an a priori compliant subset (CPAP use 3.5 hours/night) DBP remained significantly lower by a magnitude of 1.9 mmHg (p = 0.042). In the other a priori subset of severely hypoxaemic patients (4% desaturation index 20/hour) there were also falls in Systolic BP (4.0 mmHg, p = 0.011), DBP (5.0 mmHg, p = 0.001) and mean arterial pressure (3.4 mmHg, P=0.023). Although all the reductions in pressures were small, data from population studies suggest such reductions may be associated with significant health benefits. In addition the effects may have been underestimated as the equipment used to measure BP may cause an arousal from sleep there by artificially elevating the night-time BP recorded. The baroreceptor function was not different between the two treatments. The urinary microalbumin was abnormal in 35% of the patient group, the reasons for this needs further investigation. The benefits found in quality of life confirmed previous studies although this is the first randomised controlled trial to show benefits in the Functional Outcomes of Sleep Questionnaire. The lack of improvement in the neuropsychological testing may reflect the tests used. The CPAP compliance on this study was less than ideal, but similar to those in other prospective studies.