This study investigated the use of regulated cyclic breath-holds to improve microcomputed tomography (microCT) imaging of small (diameter, less than 1 mm) mouse lung tumors in vivo. Two novel techniques that use a modified small-animal ventilator were examined and compared with a previously used respiratory gating microCT technique and a free-breathing microCT technique. Two mice were scanned with each of these 4 microCT techniques (voxel size, 92 microm). The appearance of small lung tumors (maximal diameter, 0.5 to 1.0 mm) and the characteristics of line profiles of the lung-diaphragm boundary were used to compare the images obtained from the 4 acquisition techniques. The use of cyclic breath-holds, synchronized with the CT exposures, led to marked improvement in the visualization of the mouse lung structure and lesion conspicuity. A secondary experiment was performed to assess the stress placed on mice by the acquisition techniques.