To evaluate the safety and cost of outpatient cardiac catheterization, we conducted a randomized trial at three hospitals of outpatient (n = 192) as compared with inpatient (n = 189) cardiac catheterization in low-risk patients. Outpatients had the following complication rates as compared with inpatients: hematoma, 12 versus 8.5 percent; numbness or weakness of extremity, 0.5 versus 1.6 percent; cold or blue extremity, 1.6 versus 1.1 percent; and acute myocardial infarction, 1.6 versus 0.5 percent. None of these differences were statistically significant. No deaths or strokes occurred in either group. Twenty-three patients (12 percent) assigned to the outpatient group required hospitalization because of complications of catheterization. In the outpatient group, the relative risk for hematoma was 1.42 (95 percent confidence interval, 0.77 to 2.29), and the relative risk for myocardial infarction within one week was 2.95 (95 percent confidence interval, 0.3 to 28.1). There were no significant differences between the two groups in whether they resumed normal activities or in the rates of rehospitalization within one week of the procedure. Total catheterization-related charges per patient were $679 lower for outpatients, with a savings in total hospital charges (including charges for subsequent therapeutic procedures) of $885 per patient. We conclude that elective cardiac catheterization as an outpatient procedure for selected patients is feasible and safe. Given the small size of our sample, however, we urge caution in interpreting these findings, since they do not exclude a small increase in complication rates with outpatient cardiac catheterization.