Thin-sheet laser imaging microscopy (TSLIM) was used to serially section five whole cochleas from 4-wk-old CBA/JCr mice. Three-dimensional reconstructions of Rosenthal's canal (RC) were produced in order to measure canal length and volume, to generate orthogonal cross sections for area measurements, and to determine spiral ganglion neuron (SGN) number. RC length averaged 2.0 mm ± 0.04 (SEM) as measured along the centroid of the canal compared to an average basilar membrane (BM) length of 5.9 ± 0.05 (SEM). RC volume averaged 0.036 mm 3 ± 0.009 (SEM). Significant increases in the radial area of RC were observed at the base (13%), middle (62%), and apex (90%) of its length. The total number of spiral ganglion neurons (SGNs) in RC in each of the five animals averaged 8626 ± 96 (SEM). SGN number increased at the expanded regions of RC. Increased area and cell number at the base and apex are likely related to extensions of the organ of Corti past the length of RC in these areas. The increase in area and cell number in the middle of the RC appears to be related to the most sensitive frequency region of the organ of Corti. Volume imaging or tomography of the cochlea as provided by TSLIM has the potential to be an efficient and accurate semi-automated method for the quantitative assessment of the number of SGNs and hair cells of the organ of Corti.