The relationship between otitis media with effusion (OME) and chronic rhinosinusitis with nasal polyposis (CRSwNP) remains unclear. We conducted a cross-sectional study of 80 consecutively presenting patients-42 males and 38 females, aged 15 to 76 years (median: 48)-who were diagnosed with CRSwNP. Our aim was to ascertain the prevalence of OME in CRSwNP patients, to determine whether the severity of CRSwNP affected OME, and to identify risk factors for OME in CRSwNP patients. The severity of CRSwNP was assessed on the basis of nasal symptoms, endoscopic nasal examinations, and Lund-Mackay staging scores. In addition to demographic data, we obtained information on each patient's history of otitis, otoscopic findings, and the results of pure-tone audiometry and tympanometry. We then compared the data between CRSwNP patients with OME (n = 20) and those without (n = 60). In the OME group, a conductive hearing loss was present in 16 patients (80.0%); all patients in the control group had normal hearing. With regard to symptoms, only rhinorrhea appeared to be more common in patients with OME than in those without, although the difference was not statistically significant. We found no significant difference in nasal polyposis grades between the two groups. Also, we found no correlations between the risk of OME and previous surgical treatment, asthma, allergy, aspirin intolerance, aspirin and/or NSAID intolerance, aspirin and/or sulfite intolerance, and aspirin-exacerbated respiratory disease. Based on our findings, we conclude that OME occurs frequently during the evolution of CRSwNP, even when the nasal disease is well controlled. This finding suggests the possible presence, in OME and/or CRSwNP, of a global inflammatory process that involves the epithelium in both the middle ear and upper airway.