The purpose of this study was to investigate the incidence of complications in pediatric spastic cerebral palsy (CP) patients weighing less than 20 kg at the time of intrathecal baclofen (ITB) pump implantation and to compare it with spastic CP patients heavier than 20 kg. Twenty-seven patients with spastic CP (14 males) treated with ITB implantation at our institution between January 2002 and January 2018 were retrospectively reviewed. Eight of the 27 patients had a bodyweight below 20 kg (group A) and 19 had weight above 20 kg (group B). Group A had a significantly more important proportion of patients with the Gross Motor Function Classification System V compared to group B (88 vs. 42%). The median follow-up was respectively 2.5 (1.8–4.6) and 4.6 (1.9–10.0) years in groups A and B. Median age at the time of ITB implantation was 7.4 (2.8–12.8) and 13.7 (6.5–16.8) years in groups A and B (P = 0.002). The proportion of patients with complications or reoperation was NS between groups A and B (P > 0.05). No postoperative infections were recorded in any of the groups. During follow-up, five patients died (63%) in group A and three (16%) in group B (P = 0.049) within 3.8 years on average after ITB implantation. ITB therapy in spastic CP patients weighing less than 20 kg seems to be as well tolerated and effective as it is in heavier (>20 kg) pediatric patients.