OBJECTIVE: The aim of this study was to assess efficacy, safety, and tolerability of highly purified cannabidiol oil (CBD) as add-on therapy for the treatment of a series of patients with infantile epileptic spasms syndrome (IESS) who were resistant to antiseizure medications and ketogenic dietary therapy. MATERIAL AND METHODS: We conducted a retrospective analysis of the medical records of 28 infants with treatment-resistant IESS aged 6 to 21 months who received highly purified CBD between July 2021 and June 2023. Data were collected on neurological examinations, EEG, Video-EEG and polygraphic recordings, imaging studies, laboratory testing, and seizure frequency, type, and duration, and adverse effects. As the primary outcome, a reduction of frequency of epileptic spasms (ES) was assessed. ES freedom was considered after a minimal time of 1 month without ES. RESULTS: Sixteen male and 12 female patients, aged 6-21 months, who received CBD for treatment-resistant IESS were included. The etiology was structural in 10, Down syndrome in seven, genetic in nine, and unknown in two. Initial CBD dose was 2 mg/kg/day, which was uptitrated to a median dose of 25 mg/kg/day (range, 2-50). Prior to CBD initiation, patients had a median of 69 ES in clusters per day (range, 41-75) and of 10 focal seizures per week (range, 7-13). After a mean and median follow-up of 15 and 12.5 months (range, 6-26 months), seven patients were ES free and 12 had a >50 % ES reduction. Five of seven patients (71 %) with Down syndrome and 3/5 (60 %) with cerebral palsy responded well. Adverse effects were mild. EEG improvements correlated with ES reductions. CONCLUSION: In this study evaluating the use of CBD in children with IESS, 19/28 (67.8 %) had a more than 50 % ES reduction with good tolerability.