In this controlled randomised double-blind parallel group study of the use of ioversol-240 and ioversol-320 in venography all studies were considered diagnostic with comparable quality in the two groups. Patient tolerance was high with mild heat observed in 7 patients in the ioversol-320 group and 1 patient in the ioversol-240 group. Assessment of pain was also comparable (2 patients in the ioversol-240 group and 1 in the ioversol-320 group). Both strengths of the contrast agent produced no clinically significant, drug related, changes in vital signs or laboratory parameters and there were no significant clinical adverse reactions.