The side-effects of contrast medium phlebography used in the diagnosis of deep vein thrombophlebitis (DVTP) were reported. The phlebograms were obtained with the patient in a semi-erect position, with an average dose of 125 ml of contrast medium infused continuously over 7-12 min. Renografin 60, or the same agent diluted to 3/4 strength, was used. The major side-effects were pain during the study and a symptom complex resembling DVTP starting 12-36 h after the study. Both were more common with the standard contrast medium (pain: 59.0% vs. 30.0%; delayed reaction: 24.0% vs. 7.5%), although the percentage of positive studies was unchanged (about 35.0%). Side-effects were more frequent and severe than previously thought, but their incidence can readily be lowered.