Background: Syndrome Z describes the interaction of obstructive sleep apnoea (OSA) with the metabolic syndrome. Purpose of study: A pilot study to determine the prevalence of syndrome Z in a teaching hospital in Singapore. Methods: Patients (age >18 years) recruited for this prospective study had to satisfy three of the following five inclusion criteria: fasting glucose .6.1 mmol/l, blood pressure >130/85 mm Hg, HDL cholesterol ,1.04 mmol/l in men and ,1.2 mmol/l in women, triglycerides >1.7 mmol/l, and a waist circumference .102 cm in men and .88 cm in women. All subjects underwent standard overnight polysomnography. Overnight fasting glucose and lipid levels were measured and baseline anthropometric data recorded. All sleep studies were scored and reported by a sleep physician. OSA was deemed to be present if the respiratory disturbance index (RDI) was >5, with mild, moderate and severe categories classified according to the Chicago criteria. Results: There were 24 patients (19 males and five females) of whom 10 were Chinese, eight Malay and five of Indian origin, with one other. Mean age was 48¡13.5 years, mean body mass index was 34.9¡6.1 kg/ m 2 and mean waist circumference was 111.3¡15.7 cm. 23 (95.8%) of the patients had OSA with a mean RDI of 39.6¡22.4 events/h with 15 patients (62.5%) in the severe category. The five patients who fulfilled all five criteria for diagnosis of the metabolic syndrome had severe OSA. Conclusion: The prevalence of OSA in our studied population exhibiting the metabolic syndrome is very high. Therefore, a polysomnogram should always be considered for this subset of patients.