Two thousand thirty-four consecutive patients presented for excretory urography within a 22-month period. Of 57 observed adverse reactions (incidence 2.8%), 54 occurred in 1219 low-risk patients injected with conventional ionic contrast media (HOCM) (incidence 4.4%) while three reactions were noted in the 815 high-risk patients receiving low-osmolar contrast media (LOCM) (incidence 0.4%). Despite strict enforcement of an unchanging list of high-risk criteria by the same pool of radiologists, LOCM use was not constant, increasing in use over time from 26.5% to 55.3% of urograms. In addition, frequency of LOCM selection increased transiently (from 33% to 52%) following a single life-threatening reaction to HOCM. Our results suggest that strict guidelines for use of LOCM are subject to loose individual physician interpretation. Physicians' perceptions of safety made it increasingly difficult to withhold LOCM and progressively more patients were included in high-risk groups.