It was recognized that overxpansion of the remaining lung might be predominant cause for reactivation of residual foci. Comparative studies were made on sixty-eight cases among 700 resections who were followed up more than one year after resection and had been known to have residual foci. All had brochographic examinations before and after operation. Following results were obtained. 1) None of factors except overexpansion could be considered to be in intimate relation with reactivation of residual foci. 2) The mode of reexpansion of the remaining lung was classified into five types. Uniform expansion and displacement of each segment were seen in very few cases. Compensatory expansion of the adjacent segment to the resected one were seen in most cases, especially displacement of segments located anteriorly was frequently encountered. 3) Diameter of peripheral bronchus was measured on both pre-and postoperative bronchograms and the ratio was calculated. It increased in proportion to the degree of displacement and expansion of the remaining lung, that is, expansion of the lung was followed by dilatation of the bronchus. 4) Diameter of the bronchus of compensatorily expanded segment was within 1.4 times compared to preoperative value in non-reactivated group and more than 1.5 times in rectivation group. 5) Reactivation of residual foci developed in the segment associated with compensatory dilatation of the bronchus. As dilatation of the bronchus was merely secodary change, it could be concluded that reactivation in the ipsilateral lung was caused by overexpansion.