The diagnostic criteria and the length of the observation period are essential factors influencing the results of treatment of maxillary sinusitis. In 198 patients (244 sinuses) with chronic maxillary sinusitis of either rhinogenous or dental etiology all patients were judged as cured or improved at the short-term control 1-3 months after completion of therapy. The long-term observation (mean 3.5 years) revealed different figures of healing. Satisfactory results after conservative therapy were seen in only 34% while the Caldwell-Luc operation gave good results in 80%. In sinusitis of dental origin, dental treatment combined with local sinus surgery was successful in 90%. In 78 sinuses investigated by sinoscopy, discrepancy between the symptoms and the endoscopic findings was seen in 14 cases (18%). Information obtained by questionnaire is therefore unreliable. In 30 sinuses operated upon with the Caldwell-Luc procedure, discrepancy between radiographic and endoscopic findings was seen in 3 cases (10%). Contributory factors, e.g. nasal polyps, dental infections and nasal allergy were found in 48 out of 84 sinuses not completely healed at the long-term control. Patients treated for chronic maxillary sinusitis must be followed up over a long period. A clinical control after 1-2 years, including sinoscopy or sinus radiographs, is recommended even in patients free from symptoms of sinusitis. Sinoscopy seems to be more reliable than sinus radiography and should be performed if the sinus radiographs show any pathology. The patients are also recommended to visit their dentists regularly, due to the close relationship between dental infections and chronic maxillary sinus diseases.