X-ray microcomputed tomography (micro-CT) with a respiratory gating system is a useful non-invasive approach to evaluate lung tumor development in living animal models. Here micro-CT was applied for the detection of lung lesions induced by a single intraperitoneal injection (250 mg/kg) of urethane in male A/J mice, at 2-week intervals from 10 to 30 weeks after carcinogen exposure. In micro-CT cross sections, lung tumor images were easily distinguished from surrounding non-tumorous tissues, the smallest detected tumor being approximately 0.5 mm in diameter. All of the urethane-treated mice (n = 15) developed lung tumors and the number of tumors developed in each mouse was 8.6 +/- 3.9. Six tumors, determined histopathologically to be adenocarcinomas, were detected, growing at different rates during the experimental period. The most aggressive carcinoma, increasing in diameter from 0.9 to 3.5 mm within 8 weeks, was a solid-type nodule with a clear tumor margin on the micro-CT imaging. Other tumors, histopathologically adenomas, grew slowly or moderately. The results provide evidence that micro-CT is a useful non-invasive imaging approach for evaluating the characteristics and growth of lung tumors in mice.