In the majority of patients admitted to an Intensive Care Unit with acute respiratory failure (ARF), the aetiology for ARF is quite evident. In a minority of patients no obvious aetiology is apparent at presentation. In this group a previously unrecognized sleep-related breathing disorder (SRBD) may be the cause of the ARF. In spite of clinical suspicion SRBD remains infrequently diagnosed in ARF also because the technology necessary for this type of diagnosis (polysomnography) is usually unavailable in Intensive Care Units. The aim of this study was to evaluate the utility of portable polysomnography system (PSGp) in a group of patients with ARF of unclear aetiology and with a clinical suspicion of SRBD. We studied a selected group of 14 patients (eight males, six females) admitted to an Intermediate Intensive care unit with varying degree of acute respiratory failure. Mean (SD) age was 57 (13) years, pH 7.28 (0.04), PaO2 5.6 (0.7) kPa), PaO2 (8.8 (1.6) kPa), Body mass index 42.7 (9.6) kg m(-2). The patients had no history of skeletal, neuromuscular or cardiovascular disease. None of them had a history of overt chronic lung diseases or had obvious respiratory tract infections. They were submitted to cardiac and respiratory functional evaluation and to nightly PSGp (VITALOG HMS 5000, Respironics Inc., Redwood City, CA, U.S.A.) which was performed in an intermediate intensive care unit. Ten subjects had obstructive sleep apnoea-hypopnoea syndrome (OSAS), with mean respiratory disorder index h(-1) (RDI) 60.1 (25.9) [in five associated with obesity-hypoventilation syndrome (OHS)]; two had central sleep apnoea with mean RDI 45 (28.3) (one with hypothyroidism and one with cerebral multiple infarctions and right hemidiaphragmatic paralysis) and two had OHS with mean RDI 12.5 (3.5). Nocturnal hypoventilation was present in almost all patients. Continuous positive airway pressure (CPAP) was effective in three patients. Eight patients needed to be treated with BILEVEL (BiPAP, Respironics Inc.) airway positive pressure in timed or spontaneous/timed modes. Two patients required intubation and mechanical ventilatory treatment. In one patient with hypothyroidism was sufficient to institute hormonal therapy. Our study shows that acute respiratory failure due to SRBD is not exceptional in an Intermediate Intensive Care Unit and that if clinical suspicion is strong, portable polysomnography may yield diagnostic confirmation and help in establishing appropriate treatment and in avoiding the invasive ventilatory treatment.