['Early presentation of congenitally deaf children for cochlear implantation is leading to surgery in younger candidates. The safety of cochlear implantation in children aged 12 months and younger is reviewed with radiologic assessment of mastoid bone anatomy and surgical outcome data.', 'Analysis of case records and temporal bone computed tomography (CT) scans with description of surgical technique in infants.', 'Chart analysis of children aged 12 months or younger at cochlear implantation. Mastoid bone anatomy was compared with older children (mean age 2 years) using CT scans.', 'Twenty-five infants received implants at 7 to 12 months of age because of meningitis (n = 4) or early detection of deafness (n = 21). Mastoid marrow content on CT scan was significantly greater in this age group (P < .001 Mann-Whitney rank sum test), but pneumatization was always adequate for safe identification of surgical landmarks. The smaller size of the mastoid bone was not restrictive. An extended postauricular approach was used in the first 11 cases and a 2.5 cm hair-line incision in the remainder. Ligature tie-down of the device was completed in all cases. No complications occurred. All are full-time implant users, except one with other neurologic sequelae of preoperative meningitis.'] Do cochlear implant surgery at 12 months of age or younger?