PURPOSE: Small uterine cervical tumors are often undetected on MRI, and undetectable tumors are usually regarded as being in their early stages. The purpose of this study was to evaluate morphological factors determining MRI detectability of uterine cervical tumors by comparing detected and undetected tumors using high resolution MRI units. MATERIALS AND METHODS: Sixty-six surgically-proven uterine cervical cancer patients were included in this study. Using a GE Signa 1.5T magnet (GE, U.S.A.) axial T1-weighted MR images and axial, sagittal, and coronal T2-weighted fast spin echo MR images were obtained with a 5 mm thickness/2 mm gap and a 512x256 matrix size. The patients were divided into two groups, tumor-detected and tumor-undetected. Maximal tumor dimension, depth of stromal invasion, and horizontal tumor spread, measured during histopathological examinations, as well as pathological stages, were compared between the two groups. RESULTS: In the tumor-detected group, the pathological stages were IB1 in 25, IB2 in 3, IIA in 10, and IIB in 5 patients; in the tumor-undetected group, the stages were IA1 in 8, IA2 in 2, and IB1 in 13 patients. Maximal tumor dimension, depth of stromal invasion and horizontal tumor spread for tumor-detected and -undetected groups were 27.4 (10-60) mm vs. 10.4 (3-40) mm (p=0.077), 11.1 (3-20) mm vs. 2.3 (0-5) mm (p<0.001), and 26.6 (8-60) mm vs. 10.4 (3-40) mm (p=0.057), respectively. CONCLUSION: Of the criteria studied, depth of stromal invasion is the most important factor in determining tumor detectability on MRI. All tumors in which this depth was more than 5 mm were thus detected; which are undetected on high resolution MRI can be regarded as early-stage tumors (stage IB1).