A number of predisposing factors (obesity, nasal obstruction, adenoidal hypertrophy, macroglossia, etc) have been related to obstructive sleep apnea syndrome (OSAS). In addition hypothyroidism and large goitres have been reported to be associated to OSAS, but this association has not been adequately studied. We describe an obese patient with euthyroid goitre associated with OSAS. The patient showed a body mass index (BMI) of 47 and a large neck with a circumference of 60 cm. The flow-volume curve demonstrated an expiratory plateau suggesting an intrathoracic upper airway obstruction. Arterial blood gas analysis results were: pH 7.39; PCO2 54.2 mmHg; P O2 47 mmHg. Nocturnal polisomnography showed an apnea/hypopnea index (AHI) of 31 episodes/hour. Upper airway collapse was overcome by a nasal continuous positive airway pressure (nCPAP) of 14 cmH2O. Weight loss obtained by a hypocaloric diet was not accompanied by any OSAS improvement. After thyroidectomy, a nCPAP of 4 cmH2O was sufficient to prevent upper airway closure. Discontinuation of nCPAP treatment for 4 consecutive nights did not determine worsening of sleep apnea symptoms, nor a worsening of overnight oxymetry. A new polysomnography carried out after 4 nights off nCPAP showed an AHI of 33 episodes/hour. OSAS should be suspected in patients with large goitres. Decisions regarding discontinuation of nCPAP treatment after thyroidectomy should be based on polisomnographic results.