Investigation and treatment of sleep apnoea/hypopnoea syndrome (SAHS) is placing increasing demands on healthcare resources. This workload may be reduced by using split-night studies instead of the standard full-nights of diagnostic polysomnography and continuous positive airway pressure (CPAP) titration. Splitnight studies involve polysomnography in the first half of the night followed, if there is an abnormal frequency of apnoeas and hypopneas, by CPAP titration for the remainder of the night.The authors' database of all patients prescribed a CPAP trial 1991±1997 was used to compare long-term outcomes in all 49 (46 accepting CPAP) patients prescribed split-night studies with those in full-night patients, matched 1:2 using an apnoea/ hypopnoea index (AHI) of 15% and Epworth score of 3 units.Classical symptoms of SAHS were the main reason for the split-night studies (n=27). There were no differences between the groups in long-term CPAP use, median nightly CPAP use (split-night 6.0 h . night -1 , interquartile range (IQR) 3.8±7.4, fullnight; 6.2 h . night -1 , IQR 3.7±7.0, p=0.9), post-treatment Epworth scores and frequency of nursing interventions/clinic visits required. The median time from referral to treatment was less for the split-night patients (13 months, IQR 11±20 months) than for full-night patients (22 months, IQR 12±34 months; p=0.003).Split-night studies, in selected patients, result in equivalent long-term continuous positive airway pressure use to full-night studies with shorter treatment times and less healthcare utilization. Eur Respir J 2000; 15: 670±675. Sleep apnoea/hypopnoea syndrome (SAHS) is a common condition, affecting 2±4% of the middle-aged population [1]. The consequences of SAHS include daytime sleepiness, altered mood and impaired cognitive function [2,3] leading to reduced quality of life [4,5] and an increased risk of accidents at work and on the road [6±8]. Many sleep centres are struggling to cope with the number of patients being referred and the financial restrictions placed on their investigation. One way of reducing the time taken to investigate and treat patients is to use splitnight studies whereby diagnostic polysomnography and continuous positive airway pressure (CPAP) titration are accomplished on the same night rather than the standard two nights consisting of a diagnostic polysomnography night and subsequent CPAP titration night (full-night patients).Despite the potential savings, split-night studies are not widely used in all countries, at least partly because of concerns about the unknown long-term outcome of these studies [9]. The few evaluations that have included clinically relevant outcomes have not used a control group [10], or, when a control group was used, the numbers were small (n=12, in each group) and the outcome evaluation was limited to mean nightly CPAP use at the initial clinic visit [11]. Therefore, a larger scale study of the value of split-night studies was performed. MethodsUsing the authors' database of 1,211 patients booked for CPAP tit...