In a long-term follow-up study (mean 3.5 years) after treatment of chronic maxillary sinusitis of either rhinogenous or dental origin, 72 sinuses in 66 patients were examined with respect to maxillo-ostial and nasal resistance. In sinusitis of rhinogenous etiology, the mean equivalent ostial diameter was pathologically narrowed (0.89 mm, in patients treated conservatively) whereas it was normal (2.51 mm) in sinusitis of dental etiology (dental treatment in combination with local sinus surgery). In the former group, all sinuses were diseased, compared with 3 out of 23 in the dental group. In order to distinguish between functional and organic ostial stenosis, the patients were also tested after physical exercise. The functional ostial diameter in healthy sinuses was significantly increased in contrast to the diseased sinuses. In one patient the narrow ostium was due to mucosal swelling only, but in the other cases was caused mainly by an organic stenosis. In sinusitis of rhinogenous etiology, a pathologically increased nasal resistance was found in 14.3%, while the corresponding figure in the dental group was 4.3%. Improved sinusal and nasal ventilation, achieved either pharmacologically or, preferably by surgical intervention, seems necessary for successful treatment of chronic maxillary sinusitis.