In order to plan an adequate treatment, it is very important to recognize the transfissural spread of lung cancer near a pleural fissure, especially in the patients who cannot bear pneumonectomy because of impaired pulmonary function. Fifteen patients with lung cancer near a fissure were examined by means of conventional CT (10 mm-thick slices) followed by high-resolution CT (HRCT). The results obtained with both techniques were compared with surgical findings. At surgery, major fissure involvement was seen in 12 cases, minor fissure was involved in 3, and 2 patients were normal. In 4 cases fissural involvement was correctly demonstrated by conventional CT scans. Neoplastic involvement of the major fissure was identified in 13 cases by HRCT scans, with only 1 false positive. Pleural fissures appear as thin and sharp lines on HRCT scans. Thus, the relationship of lung cancers to fissures is better identified and the accuracy of the method in defining neoplastic spread is higher. However, the relationship of a neoplastic mass to the minor fissure is difficult to evaluate, because the fissure is roughly parallel to the scanning plane.