PURPOSE. High-resolution magnetic resonance imaging (MRI) can now directly demonstrate innervation to extraocular muscles and quantify optic nerve size. A quantitative MRI technique was developed to study the oculomotor nerve (CN3) and applied to congenital fibrosis of extraocular muscles (CFEOM) and congenital oculomotor palsy. METHODS. The subarachnoid portions of the CN3s were imaged with a 1.5-T MRI scanner and conventional head coils, acquiring heavily T 2-weighted oblique axial planes 1-mm thick and parallel to the optic chiasm. Thirteen normal subjects, 14 with CFEOM, and 3 with congenital CN3 palsy were included. Digital image analysis was used to measure CN3 diameter, which was correlated with motility findings. RESULTS. In CFEOM, CN3 diameter was bilaterally subnormal in eight subjects, unilaterally subnormal in three subjects, and normal in three subjects. Mean ± SD CN3 diameter in CFEOM was 1.14 ± 0.61 mm, significantly smaller than the diameter in normal subjects, which measured 2.01 ± 0.36 mm (P < 0.001). CN3 diameter variably correlated with clinical function. One subject with congenital CN3 palsy showed bilateral CN3 hypoplasia, but CN3 diameter was normal in two other subjects with congenital CN3 palsy. CONCLUSIONS. Unilateral or bilateral hypoplasia of CN3 is quantitatively demonstrable using MRI in many cases of CFEOM and occasionally in congenital CN3 palsy. Variations in CN3 diameter in CFEOM and congenital CN3 palsy suggest mechanistic heterogeneity of these disorders that may be clarified by further imaging and genetic studies.