The frequency of side effects of a nonionic contrast agent (iohexol) was studied in 463 consecutive patients who underwent venography for clinically suspected deep-vein thrombosis (DVT) and compared with the frequency of adverse reactions of another series in which patients received either the same contrast material or a high-osmolar ionic compound. Minor side effects, including local pain and discomfort, nausea and vomiting, dizziness, skin reactions, superficial phlebitis, and edema, occurred in 83 patients (17.9%; 95% confidence interval [CI], 15%-22%). The only serious adverse reaction (bronchospasm) was seen in two patients (0.4%; 95% CI, 0.1%-1.4%). Postvenographic thrombosis confirmed by means of repeat venography occurred in one of 41 consecutive patients with a previous normal venogram (incidence, 2%; 95% CI, 0%-13%). The frequency of side effects appears to be significantly less than when conventional high-osmolar contrast agents are used. Use of iohexol for venography is associated with minor side effects in approximately one-fifth of patients, and serious adverse reactions necessitating therapy are rare.