OBJECTIVE. The purpose of the study was to determine the correlation between frontonasal duct abnormalities (narrowing or obstruction caused by hypertrophic mucosa) and frontal sinusitis. This study was based on the hypothesis that abnormalities of the frontonasal duct cause frontal sinusitis by impairing normal drainage of the sinus. MATERIALS AND METHODS. CT studies of 198 consecutive patients with clinical diagnoses of chronic sinusitis were reviewed retrospectively. Criteria for inclusion were (1) no history of sinus surgery or facial trauma and (2) absence of polyps at rhinoscopy. As 37 frontal sinuses were undeveloped, a total of 359 sinuses were evaluated. CT scans were obtained in oblique axial and coronal planes. The following CT features were assessed: (1) the frequency of detection of the frontonasal duct, (2) the appearance of the frontonasal duct: normal vs abnormal (narrowed or obstructed), and (3) the correlations between abnormalities of the frontonasal duct and frontal sinusitis. RESULTS. The frontonasal duct was detected in all 359 cases, either in both CT planes (81%) or only in the axial oblique plane (19%). In 267 (74%) of 359 cases, the duct appeared normal; among these, isolated frontal sinusitis was detected in five cases (2%). In 92 (26%) of 359 cases, the duct was abnormal; it was narrowed in 18 cases (5%) and obstructed in 74 cases (21%). Frontal sinusitis was noted in 78 (85%) of the 92 cases of frontonasal duct abnormalities. The sensitivity and specificity of the correlations between frontonasal duct abnormalities and frontal sinusitis were 98% and 85%, respectively. CONCLUSION. Because our results show a strong correlation between abnormalities of the frontonasal duct and frontal sinusitis, it seems highly probable that abnormalities of the frontonasal duct cause frontal sinusitis.