Purpose : To evalvate the usefulness on a CT chest scan, of the anterior junction line as an anatomical landmark to distinguish the right middle and the right upper lobeMaterial and Methods : We found that the anterior junction line has a constant anatomical relationship with the right upper and middle lobe, and with this in mind, analysed connvcntional CT films of 86 patients with normal lung(group A) and 30 with architectural distortion (group B). On a series of slices, we compared the location of slice 1 with that of slice 2(slice 1 : the slice which includes the lowest portion of the anterior junction line, slice 2 : the initial slice, in which the right middle lobe occupies the whole of the lung anterior to the right major fissure).Result : In group A(n = 86), the right upper lobe, as seen in the anteromedial zone of slice 1, was present in 83 cases(96.5%). The right upper lobe on slice 1 was absent in two cases(2.3%) in which a minor fissure was almost completely abent. In group B(n = 30), the right upper lobe on slice 1 was absent in 19 cases(63.3%).Conclusion : We suggest that on a CT chest scan, the anterior junction line can be used as an anatomical landmark in the differentiation of the right middle from the right upper lobe, and as an indicator of the presence of architectural distortion.