This study is a prospective, randomized, placebo-controlled, double-blinded clinical study. The balloon sinuplasty treatment is compared to placebo effect in chronic or recurrent maxillary sinusitis. The purpose is to find out the efficacy of balloon sinuplasty of maxillary sinuses and the patients who get the best benefit from balloon sinuplasty. The investigators also want to find out if balloon sinuplasty of maxillary sinuses improves Eustachian tube dysfunction. The study is executed in the Department of Ear and Oral Diseases, Tampere University Hospital. 120 patients who have either recurrent acute or chronic rhinosinusitis without nasal polyposis and whose sinusitis is limited mainly into maxillary sinuses, are recruited in the study. The patients are allocated either into the chronic maxillary sinusitis (60 patients) or the recurrent maxillary sinusitis (60 patients) branch of the study. Then, the patients are consecutively randomized into two treatment groups in the proportion of 1 to 1: A) Balloon sinuplasty group and B) Placebo group. The treatment is performed according to the patient's group status and all the patients are followed 12 months postoperatively. The effect of balloon sinuplasty treatment in chronic maxillary rhinosinusitis is going to be measured primarily with the Sino-Nasal Outcome Test (SNOT-22). In recurrent acute maxillary rhinosinusitis, the effect of balloon sinuplasty treatment will be measured primarily with the number of acute rhinosinusitis infections. Besides this, number of antibiotic treatments and sick leaves are inquired, Visual Analog Scale (VAS) and Eustachian Tube Dysfunction Questionnaire (ETDQ-7) are used, rhinomanometry, Cone Beam Computed Tomography (CBCT), tympanometry, tubomanometry, nasal endoscopy and general evaluation of the patients clinical ORL-status are performed. Also pre- and postoperative biopsies are taken from the middle turbinates.