Spinal anesthesia is increasingly used in adolescents. However, the anesthesia provided by bupivacaine alone may be too short for the planned surgery. The addition of clonidine 2 microg/kg to bupivacaine provides a prolonged anesthetic action but may be associated with hypotension. In the present study, we investigated the efficacy and safety of intrathecal clonidine 1 mug/kg in adjunction to bupivacaine in spinal anesthesia in adolescents. Eighty-three adolescents, 51 males, aged 10-15 yr, scheduled for orthopedic surgery were enrolled in this placebo-controlled, randomized study. Patients were given spinal anesthesia either with plain 0.5% isobaric bupivacaine 0.2-0.4 mg/kg or bupivacaine with clonidine 1 microg/kg. The duration of sensory block was the primary outcome measure. Clonidine prolonged the duration of both the sensory and motor block. The time to regression of sensory block by two dermatomes was 136 (mean) (sd, 56) min in the adolescents with clonidine versus 107 min (sd, 42) in the controls (95% CI for diff: 5-53 min, P = 0.02). The time to full recovery of motor block was 251 min (sd, 79) in the adolescents with clonidine versus 181 min (sd, 59) in the controls (95% CI: 39-103 min, P = 0.001). Time to the first dose of rescue analgesia was longer in the adolescents with clonidine, 461 min (sd, 147) versus 330 min (sd, 138) in the controls (95% CI: 53-207 min, P = 0.01). There was no difference in the frequency of hypotension or bradycardia between the groups. In adolescents, clonidine 1 microg/kg prolonged the duration of sensory block achieved with bupivacaine by 30 min and postoperative analgesia by 120 min without severe adverse events.