AbstractCostello syndrome (CS) was initially described by Costello in 1971; it is caused by a germline mutation in HRAS proto‐oncogene. The aim of the present study was to evaluate the respiratory activity during sleep in a group of subjects with CS. We studied 10 consecutive patients, 4 males and 6 females, aged 3–29 years, affected by CS. All patients underwent clinical, neurological, otholaryngologic and radiologic evaluation, and a full‐night polysomnography in the sleep laboratory. Polysomnography showed that seven patients presented a relevant number of respiratory events of obstructive type during sleep. The apnea‐hypopnea index (AHI) ranged from 0 to 19.2 events per hour (mean index = 7.5 ± 6.9 events/hr). In one patient AHI was not evaluable because of tracheostomy. Apnea induced mild or moderate hemoglobin desaturations (mean of lowest SpO2 values = 85.4 ± 5.5%). Only sporadic respiratory pauses of central type were observed (mean number of central apnea per study: 7.2 ± 6.8 events/hr). Sleep structure was fragmented, with a high number of awakenings (mean number of awakenings was 13.2 ± 8.1; of these, 4.8 ± 2.5 lasted longer than 2 min). In all patients, otolaryngologic and radiologic observations revealed one or more sites of narrowing in the upper airways. Our results suggest that Costello patients have a high prevalence of obstructive sleep‐related respiratory disorders, which need to be assessed by means of polysomnography. © 2006 Wiley‐Liss, Inc.