Purpose : To evaluate the distance between both upper lobe bronchi on lateral radiographs and its change in left upper or lower lobe collapse. Materials and Methods : 144 true lateral radiographs were analyzed on which both upper lobe bronchi were clearly identified. They included 116 normal cases, 11 cases of left upper lobe collapse, 13 of left lower lobe collapse, and 4 cases of left lower lobe lobectomy. Line A was drawn parallel to the vertebral end plate through the upper margin of the left upper lobe bronchus. Line B was drawn parallel to line A through the upper margin of the right upper lobe bronchus. The shortest distance between line A and line B was measured as the distance between both upper lobe bronchi. Results : In normal cases, the mean value of the distance was 2.19cm $pm$ S.D. 0.37cm on right and 2.16cm $pm$ S.D. 0.40cm on left lateral radiographs ; these results were not significantly different(P=0.79). In cases of collapse, the mean value of the distance was 0.43cm $pm$ S.D.0.99cm in upper lobe collapse and 3.56cm $pm$ S.D. 0.72cm in lower lobe collapse, results which were significantly different from those of normal cases(P<0.01). In eight cases(73%) of left upper lobe collapse, the distance was less than 1cm and in 10 cases(77%) of left lower lobe collapse, the distance was more than 3cm. Conclusion : The distance between both upper lobe bronchi varies markedly in case of lobar collapse. A distance of less than 1cmsuggests collapse of the left upper lobe and a distance more than 3cm suggests collapse of the left lower lobe.