BACKGROUND AND PURPOSE: Prior studies have evaluated cochlear length using CT to select the most suitable cochlear implants and obtain patient-specific anatomy. This study aimed to test the accuracy and reliability of cochlear lateral wall length measurements using 3D MR imaging. MATERIALS AND METHODS: Two observers measured the cochlear lateral wall length of 35 patients (21 men) with postlingual hearing loss using CT and MR imaging. The intraclass correlation coefficient (with 95% confidence intervals) was used to evaluate intraobserver and interobserver reliability for the 3D cochlear measurements. RESULTS: The mean age of the participants was 39.85 (SD, 16.60)?years. Observer 1 measured the mean lateral wall length as 41.52 (SD, 2.25)?mm on CT and 41.44 (SD, 2.18)?mm on MR imaging, with a mean difference of 0.08?mm (95% CI, ?0.11 to 0.27 mm), while observer 2 measured the mean lateral wall length as 41.74 (SD, 2.69)?mm on CT and 42.34 (SD, 2.53)?mm on MR imaging, with a mean difference of ?0.59?mm (95% CI, ?1.00 to ?0.20 mm). An intraclass correlation coefficient value of 0.90 (95% CI, 0.84?0.94) for CT and 0.69 (95% CI, 0.46?0.82) for MR imaging was obtained for the interobserver reliability for the full-turn cochlear lateral wall length. CONCLUSIONS: CT-based 3D cochlear measurements show excellent intraobserver and interobserver reliability, while MR imaging?based lateral wall length measurements have good-to-excellent intraobserver reliability and moderate interobserver reliability. These results corroborate the use of CT for 3D cochlear measurements as a reference method and demonstrate MR imaging to be an alternative acquisition technique with comparably reliable results.