Objective: To determine if the intrathecal delivery of baclofen will decrease spastic hypertonia caused by brain injury. Patients: Eleven patients more than 1 year after their brain injury with disabling lower extremity spastic hypertonia. Setting: University tertiary care outpatient rehabilitation clinic. Design: Patients were a consecutive sample randomized in a double blind, placebo-controlled crossover study. Interventions: Bolus intrathecal injection of either normal saline or 50 μg baclofen. Main Outcomes Measures: Data for Ashworth rigidity scores, spasm scores, and deep tendon reflex scores were collected for both the upper extremities (UE) and lower extremities (LE). Changes over time were assessed via Friedman's test. Differences between the placebo and active drug at any given time were assessed via Wilcoxon signed-rank. Results: Four hours after injection with the active drug (maximum effect) the average LE Ashworth score decreased from 4.2 ± 0.8 (SD) to 2.2 ± 0.6 (p = .0033), spasm score from 3.1 ± 1.0 to 1.0 ± 0.7 (p = .0032), and reflex score from 3.3 ± 0.5 to 1.0 ± 1.3 (p = .0033). The average UE Ashworth score decreased from 3.3 ± 1.3 to 1.9 ± 0.8 (p = .0033), spasm score from 1.8 ± 1.3 to 0.6 ± 1.0 (p = .007), and reflex score from 2.7 ± 0.5 to 1.7 ± 0.6 (p = .0111). No trend was observed over time with placebo administration. There were significant reductions in the average for LE Ashworth (p < .0001), spasm (p < .0001), and reflex (p < .0001) scores and for UE Ashworth (p < .0001) and spasm (p < .0004) scores observed over 4 hours (maximum effect) with active drug administration. No significant differences were noted between the active drug and placebo groups before administration in LE and UE Ashworth, spasm, or reflex scores. There were significant differences between the active drug and placebo groups at 4 hours after administration for LE and UE Ashworth, spasm, and reflex scores (p ≤ .0272). Conclusion: Intrathecal injection of baclofen is capable of reducing the spastic hypertonia associated with brain injury.