A prospective study of acute and late reactions to low-osmolal contrast media was conducted in 4,875 patients. The contrast medium was injected intravenously in 4,417 patients and intraarterially in 458 for CT, urography or angiography. In the i.v. group only nonionic contrast media, iohexol or iopamidol, were used and in the intraarterial group both ionic and nonionic contrast media, ioxaglate and iohexol, were used. The patients completed 2-phase questionnaires, the rate of reply being 88%. In the i.v. group the incidence of acute reactions was 1.2% and that of late reactions was 4.7%. In the intraarterial group the incidence of acute reactions was 7.4% and the incidence of late reactions was 4.8%. Women and patients from under 40 to 60 years of age were the most susceptible to late reactions. The risk factors for late reactions are allergy, medicine allergy, previous adverse reaction to contrast medium and other diseases including diabetes mellitus, heart-, liver- and kidney diseases.