Cervical myelography with iohexol via C1-C2 puncture was performed in 30 patients in two medical centers using a concentration of 240 mg l/ml. The study demonstrated iohexol to be a safe contrast medium without significant changes in neurologic and physical examination, vital signs, electrocardiogram, or hematologic or blood chemistry parameters. Fifteen patients had electroencephalograms (EEGs); two were abnormal. In one patient the baseline EEG demonstrated nonspecific slow waves in the temporal regions bilaterally that remained unchanged after myelography. In the second patient, transient changes in the left hemisphere during either hyperventilation or photic stimulation on postmyelographic EEG had not been present on the baseline recording. The relation of these changes to the drug remains unclear. Iohexol was found to be an efficacious myelographic contrast agent, with good to excellent myelograms in 93% of cases. Headache occurred in 13% and nausea in 3%.