Aim: To measure changes in children with severe spastic cerebral palsy (CP) after continuous intrathecal baclofen (ITB) infusion over 18 months and to compare the results with those of a comparison group awaiting treatment. Method: Thirty-eight children with severe spastic CP considered suitable for ITB were assessed when first seen, just before insertion of an intrathecal pump, and 9 months and 18 months later. Eighteen children waited around 9 months for a pump (group 1: nine males, nine females; mean age 9y 11mo [SD 3y 7mo], nine in Gross Motor Function Classification System [GMFCS] level IV, nine in level V). This baseline period was used as a control for comparison with the first and second 9-month periods after the pump for the remaining 20 children (group 2: 11 males, nine females; mean age 10y 2mo [SD 3y 1mo], nine in GMFCS level IV, 11 in level V). The main outcome measure was the Pediatric Evaluation of Disability Inventory (PEDI); other assessments were of function, ease of care, quality of life, and costs of new equipment. Results: No significant change was found in the PEDI between group 1 while awaiting treatment and group 2 in the two periods afterwards, nor in the Lifestyle Assessment Questionnaire or the cost of new equipment. Significant changes were found in group 2 in the first 9 months according to the modified Ashworth score (difference between mean values for groups -1.7, standard error 0.58; p = 0.008), Penn Spasm score (-1.3, 0.37; p = 0.001), mean joint range of movement (8.3 degrees;, 2.8; p = 0.005), and Caregiver Questionnaire (-19.7, 5.1; p = 0.01), and in the second 9 months for the Modified Ashworth Scale score (-0.62, 0.12; p = 0.001). Interpretation: ITB in children with severe spastic CP over the first 18 months improves their quality of life in terms of comfort and ease of care. It has less effect on function, participation in society, or the overall cost of new equipment.