Authors retrospectively analysed the diagnostic values of oblique tomography of hilum over simple chest PA & AP tomography in 129 cases at Seoul National University Hospital during 3yrs from may 1979 to may 1982. The results are as follows : 1. Among 129 cases, 42 cases were lung cancer, 24 cases were tuberculosis, 22 cases were prominent pulmonary artery, 10 cases were metastasis, & ect. 2. In 42 cases of lung cancer. 1) Diagnostic accuracy of oblique tomography in the evaluation of hilar lymphadenopathy was 92%, that of AP tomography & simple chest PA were 67% & 54% respectively. 2) Diagnostic accuracy of oblique tomography in the evaluation of endobronchial lesion was 95%, that of AP tomography & simple chest PA were 24% & 14% respectively. 3. It is suggested that in lung cancer if there is no evidence of hilar enlargement in simple X-ray, 55$^{circ}$ oblique tomography is recommended for evaluation of hilum, or if hilar enlargement is shown in simple X-ray, then AP tomography or CT is recommended for evaluation of mediastinum.