The study objective was to assess the efficacy and patient acceptance of ketorolac as an alternative to meperidine for the treatment of severe musculoskeletal low back pain (LBP). A double blinded prospective trial in a convenience sample of patients G18 years of age presenting to an urban university hospital emergency department (ED) was conducted over a 19-month period. Patients were included if the pain was musculo- skeletal in origin and was severe enough to warrant parenteral analgesics. Patients were randomized to receive 1 mg/kg meperidine intramuscularly (IM) or 60 mg ketorolac IM. Pain intensity was measured pre-administration and at 60 minutes via a 100 mm Visual Analog Scale (VAS). Outcomes measured at 60 minutes were pain intensity decrease (PID), patient satisfaction, rescue analgesia requirement, sedation level, and adverse effects. Clinically significant pain reduction was defined as a PID of at least 13 mm or a reduction in pain of least 30%.