Cervical spinal cord abnormalities are often unapparent on myelographic studies, because no normal values for cervical spinal cord diameter are currently available. The purpose of this study was to establish, myelographically, the normal sagittal diameter of the cervical spinal cord in large and small breed dogs and its relationship to the sagittal diameter of the vertebral canal and sagittal height/length of the corresponding vertebral bodies. Forty-one adult dogs underwent cervical radiography and myelography. Spinal cord and vertebral canal sagittal diameter, vertebral body height at C2 to 5, body length at C3 to 5, and dorsal spine length of C2 were measured on lateral views. Ratios of spinal cord:vertebral canal diameter, spinal cord:body height, and spinal cord:body length/spine were calculated, and a normal range was determined for small and large breed dogs. The spinal cord:vertebral canal ratios showed that small breeds have a higher cervical cord-to-canal ratio than large breeds. The mean values and ranges of 14 ratios are reported. The ratios of spinal cord:body length at C2 to 4 in small breeds and spinal cord:body height at C3 to 5 in large breeds were found to be the most accurate for assessing spinal cord sagittal diameter. These normal ranges would allow quantitative and objective evaluation of the cervical spinal cord by myelography and early identification of dogs with altered spinal cord diameter, which could be further evaluated by means of alternative imaging techniques.