The analgesic efficacy and safety of parenteral butorphanol and meperidine were compared. The double-blind, randomized study involved 120 patients presenting with moderate to severe renal colic. Pain intensity and pain relief were evaluated by trained observers at fixed time intervals for four hours after study drug was administered. Eighty-three patients with documented upper urinary tract calculi were evaluated for efficacy. Butorphanol 4 mg was significantly more effective than butorphanol 2 mg and was equivalent to meperidine 80 mg. Overall efficacy assessments were "good" or "excellent" for 87 per cent, 72 per cent, and 85 per cent, respectively. There were no significant differences in side effects among treatment groups in the 83 evaluable and 27 inevaluable patients receiving study drugs. Butorphanol was effective and well-tolerated in this patient population, with important advantages over opiate analgesics.