BACKGROUND: The prolonging effects of oral clonidine premedication on spinal anesthesia are known. We hypothesized that intravenous clonidine might have the same effect even if administered after an intrathecal local anesthetics injection. METHODS: To assess the prolonging effect of intravenous clonidine on spinal anesthesia, we designed a double-blinded, placebo-controlled, prospective study. Patients scheduled for orthopedic surgery were studied. All patients received 12 mg of hyperbaric bupivacaine intrathecally and were allocated to three groups. Group 1 (n = 26) was the control. Group 2 (n = 26) and Group 3 (n = 26) received 3mug/kg intravenous clonidine at the same time and 50 minutes after intrathecal hyperbaric bupivacaine injection. Sensory block was evaluated by pinprick and the duration was defined as the time needed for regression to L1 dermatome. Duration of motor blockade was defined as the time needed for recovery of knee flexion. RESULTS: The duration of sensory block was longer in Group 2 (196+/-42 minutes) and 3 (179+/-41 minutes) than Group 1 (125+/-25 minutes). The Duration of motor blockade was longer in Group 2 (153+/-26 minutes) than Group 1 (131+/-29 minutes). The lowest heart rate and mean blood pressure were not different among the groups. CONCLUSIONS: Intravenous clonidine after an intrathecal hyperbaric bupivacaine injection prolonged spinal anesthesia without added complications.