‘Overlap Syndrome‘ is togetherness of Obstructive Sleep Apnea Syndrome(OSAS) with Chronic Obstructive Pulmonary Disease(COPD) and asthma.We aimed to determine frequency of COPD and Asthma in OSASpatients and effect of these diseases on quality of sleep and severity of OSAS.Files of patients evaluated in the sleep laboratory in January 2005-January 2010 were analysed.All cases were examined for COPD, Asthma and severe daytime sleepiness according to Epworth Sleepiness Scale(ESS).Pulmonary function tests and polysomnographic tests were administered to all.For diagnosis of COPD, GOLD criteria;for EEG scoring, Rechtschaffen - Kales criteria and for respiratory scoring, AASM 1999 criteria were used.Out of 998 cases 98.2% were diagnosed as OSAS, 11.1% COPD,6,6% asthma.11.2% of OSAS cases had COPD.Age, frequency of male sex and ESS were significantly higher in patients with OSAS and COPD than patients with OSAS-without COPD,no significant differences in terms of BMI and sleep parameters were found.In OSAS+asthma cases(6.7%); BMI, frequency of female sex and ESS were found to be significantly higher than in patients with OSAS-without asthma,no statistical differences were found in terms of sleep parameters. In older male patients with OSAS;COPD is more frequent than in younger female patients.Asthma is more frequently seen in OSAS than general population.In asthmatics,sleep efficiency is lower;thereby ESS is higher.No relation between severity of OSAS and presence of COPD or asthma was determined.In patients with OSAS,symptoms of COPD and asthma must be questioned and pulmonary function tests administered.