A single, open trial was conducted in 40 patients to evaluate the diagnostic efficacy and safety, in cranial computed tomography, of ioversol (350 mgl/ml), a new nonionic, monomeric, low-osmolality contrast medium. Ioversol is characterized by a hydrophilicity which is not only the highest of all nonionic agents available to date, but also evenly distributed among the various sides of the benzene ring. Diagnosis was possible in 100% of cases with a mean degree of certainty of 90.8%. Six minor adverse reactions requiring no treatment were recorded, of which two were observed by the investigator and four reported by the patients. No pain sensation was found and heat sensations were of minor intensity. Ioversol 350, which showed good diagnostic efficacy and proved to be well tolerated, is therefore suitable for cranial computed tomography at a mean dose of 1 ml/kg.