OBJECTIVES: The purpose of this study is to determine whether there is an increased prevalence of OLD in patients with obstructive sleep apnea (OSA). METHODS: The prevalences of COPD, asthma, and COPD combined with asthma (ICD-9 coding) were compared between 1497 adult OSA patients and a 1489 control patients, matched for age, gender, geographic location, and primary care physician.  Co-morbidity burden was assessed using the Charlson Co-morbidity Index (CCI).  The prevalences of specific co-morbidities were measured in the OLD groups between patients with OSA and the matched control group. RESULTS: COPD, asthma and COPD combined with asthma were found to be more prevalent among OSA patients compared to the matched controls.  Prevalences among patients with and without OSA respectively were: COPD: 7.6%, 3.7 %(P<.0001); asthma: 10.4%, 5.1 % (P<.0001), COPD plus asthma: 3.3%, 0.9% (p<.0001).  CCI was greater for OSA patients (2.3+ 0.2) than for controls (1.9+ 1.8; p<.0001). These trends held for all severity ranges of OSA. For patients with COPD, OSA was associated with greater prevalence of diabetes (50.9% vs. 22%, p=0.004) and obesity (78.9% vs. 31.7%, p<0.0001). Patients with OSA and COPD were characterized by more severe hypoxia at night compared with the OSA patients without OLD. CONCLUSIONS:  OSA was associated with increased prevalence of OLDs. This was true for COPD, asthma and patients with both diagnoses.  Further, increased prevalence rates of OLD were found in all ranges of OSA severity.