Increasing numbers of patients are undergoing diagnostic catheterization as outpatients; however, a small proportion of patients requires hospital admission following the procedure. Unplanned admissions after consecutive outpatient cardiac catheterizations performed during 1 year were prospectively reviewed to determine the incidence of and reasons for admission. Among 847 patients undergoing outpatient cardiac catheterization, 130 patients (15%) required hospital admission after the procedure. Admitted patients were divided into four groups: patients undergoing immediate percutaneous transluminal coronary angioplasty (PTCA) (Group 1; 33%), patients with severe cardiac disease requiring urgent intervention (Group 2; 48%), patients suffering complications or hemodynamic instability (Group 3; 15%), and patients whose procedures were completed too late to allow same-day discharge (Group 4; 4%). Patients over 65 were more likely to require admission and women were more likely to be admitted with complications or hemodynamic instability. Findings are compared with results of other outpatient series, and implications regarding appropriate setting for outpatient catheterization are discussed.