Low osmolar contrast media (CM) cause less hemodynamic and electrical changes than conventional CM, and many cardiologist have switched to the newer agents. Recently, concerns about a possible increased frequency of thromboembolic complications with low osmolar CM have been raised. This study compared two low osmolar heparinized CM, one ionic (Hexabrix) and the other nonionic (Isovue), in 500 consecutive patients undergoing either diagnostic catheterization (417) or coronary angioplasty (83). Adverse effects were uncommon (37 patients), but were more frequent with Hexabrix (12%) than Isovue (3%); p less than 0.05. The most common adverse effects were nausea or vomiting. Death, related to abrupt occlusion of a severe left main stenosis occurred in one patient who received Hexabrix, and acute pulmonary edema occurred in one patient who received Isovue. No apparent thromboembolic complication occurred. In conclusion, both low osmolar CM were well tolerated, but Hexabrix caused more transient nausea or vomiting. Both CM were safe and the incidence of thromboembolic complications when prospectively looked for was very low.