Sixty unpremedicated outpatients undergoing elective extracorporeal shock wave lithotripsy using an unmodified Dornier HM-3 lithotriptor were randomly assigned to receive an intravenous infusion of either alfentanil or ketamine as an adjuvant to midazolam for sedation and analgesia. Although both drug regimens allowed the maximal number of shock waves and energy level, the alfentanil group had significantly better calculi fragmentation (78% vs. 50% of patients with fragments < 2 mm). Ketamine infusion provided superior intraoperative cardiorespiratory stability; however, it was associated with more disruptive movements (22 vs. 5) and dreaming (35% vs. 5%) during the procedure (P < 0.05). Postoperatively, confusion also occurred more frequently in the ketamine-treated patients (31% vs. 5%, P < 0.05). Alfentanil infusion was associated with more episodes of hemoglobin oxygen desaturation to < 90% (12 vs. 2, P < 0.05), itching (23% vs. 4%, P < 0.05), and ability to recall intraoperative events (45% vs. 12%, P < 0.05). The incidence of postoperative nausea was decreased (not significantly) in the alfentanil group (32% vs. 54%). The mean anesthesia time was similar in both groups; however, discharge times (means ± standard deviations) were-shorter in the alfentanil group (142 ± 42 min vs. 161 ± 31 min, P = 0.05). These data suggest that although both techniques proved effective for anesthesia in outpatients undergoing immersion lithotripsy, alfentanil is superior to ketamine as part of a sedative-analgesic technique because of the improved recovery profile and calculi fragmentation.