Purpose : To predict pulmonary arterial hypertension by comparing the extent of honeycombing lesion with thediameter of main pulmonary artery (MPA). Materials and Methods : We retrospectively reviewed 21 patients (20men,one woman) who showed honeycombing lesion on HRCT, and also analysized a control group of 30 (29men, one woman)who had no cardiopulmonary symptom and no abnormality on chest CT. On Chest HRCT, we scanned at four levels ［1)aortic arch, 2) bifurcation of bronchus, 3) mid-portion of left ventricle, 4) dome of right diaphragm］, and tworadiologists then evaluated the proportion of honeycombing lesion within the entire lung field. According to theextent of this, we divided the patients into five groups ［<20% (I), 20-39% (II), 40-59% (III), 60-79% (IV), $geq$80% (V)］, and then divided Group I into two subgroups ［<10 % (I$mid$1), 10-19% (I-2)］. Using contrast-enhancedroutine CT, the main pulmonary artery (MPA) was measured at its the widest diameter perpendicular to the long axisat the level of its bifurcation. Result : In group I (n=11) MPA was 28.4$pm$2.6mm ; in group II (n=6), it was31.7$pm$2.0mm ; in group III (n=2), 33.5mm ; in group IV (n=1), 33mm ; and in group V(n = 1), 39mm. In subgroup I-1MPA was 26.1$pm$1.9mm, and in subgroup I-2, it was 29.8$pm$1.6mm. In the control group, MPA was 25.1$pm$ 2.0mm.Conclusion : The extent of honeycombing lesion is closely related to the change in diameter of the MPA.Honeycombing lesion extending over more than 10% of the lung field may lead to pulmonary arterial hypertension.