Various definitions have been proposed for the term 'chronic sinusitis' but too often reports on chronic sinusitis lack a definition. In order to improve the diagnosis and treatment of this disease, and also to facilitate comparisons between clinical investigations, definitions of 'chronic maxillary sinusitis' of either rhinogenous or dental origin are proposed. It is suggested that the differential diagnosis should be based upon the duration of symptoms, ENT and dental examinations, sinus radiographs and/or sinoscopy, and results of conservative treatment. In a study conducted over a 5-year period, 198 patients with 244 affected sinuses fulfilled the proposed criteria. The incidence of chronic maxillary sinusitis in the population was 0.02%. A dental cause was found in 40.6% of the sinuses. The dental cause could be confirmed by routine dental examination in only 43 of 99 cases, while an extended maxillo-dental examination was conclusive in the other cases (56/99). Marginal periodontitis was found as frequently as periapical granuloma and together they constituted 83% of all dental causes. Sinusitis of dental origin was rare before the age of 30. Every sixth patient was found to have nasal polyposis. When related to the number of affected sinuses, the incidence of nasal polyposis was 13.1% in sinusitis of dental origin and 23.4% in that of rhinogenous origin. The importance of close cooperation between the ENT specialist and the oral surgeon is stressed.