Postoperative pain after total knee arthroplasty (TKA) is severe, and achieving adequate analgesia remains a clinical challenge. We tested the hypothesis that, in patients having unilateral TKA under intrathecal (IT) anesthesia, the addition of a femoral nerve block would provide superior analgesia when compared with IT morphine and demonstrate fewer adverse side effects. In a single-blinded and controlled trial, 41 ASA I-III patients undergoing unilateral TKA were randomized into 2 groups. Both groups received 15 mg of IT hyperbaric bupivacaine for the surgical anesthetic. Group ITM received 250 microg of IT morphine and group FNB received an ultrasound-assisted femoral nerve block with 40 mL of 0.5% ropivacaine, 5 microg/mL of epinephrine, and 75 microg of clonidine. At 1, 2, 4, 6, 12, and 24 h postoperatively, we measured visual analog scales for pain, cumulative IV morphine consumption, hemodynamics, and side effects. There were no statistically significant differences in morphine consumption, pain at rest, or pain with movement. However, group FNB had fewer perioperative side effects including nausea, vomiting, and pruritus (P < 0.05 for each event). This corresponded to a decrease in patient satisfaction in group ITM, in which 20% of the patients rated their experience as "unsatisfactory" (P < 0.05). We conclude that, in comparison with IT morphine, a single injection femoral nerve block provides equivalent analgesia but with a significant reduction in side effects for patients having TKA under bupivacaine intrathecal anesthesia.