Objective. To assess long-term efficacy, safety and functional benefit of intrathecal baclofen for severe spinal spasticity. Materials and Methods. This prospective multicenter study was performed in two stages: the first one consisted of an intrathecal bolus injection of baclofen, and the second of a continuous intrathecal baclofen infusion by means of an implantable pump. The sample consisted of 72 adult patients with severe spinal spasticity. Sixty-four were implanted and followed for 36 months. Muscular tone, spasms, and functional scales were evaluated before and periodically after administration of the drug, with a follow-up period of 36 months. Results. A very significant decrease in tone and spasms was observed in all cases (p < 0.001). Tolerance appeared during the first 12 months, increasing doses from a mean initial dose of 83.2 μg (range 25-200 μg) to a mean final dose of 270 μg (range 25-800 μg). Later on, efficacy remained stable, except in cases of mechanical problems of the infusion system.