Cannabidiol (CBD) is a nonpsychoactive derivative of cannabis. Studies indicate that it is safe and effective in treating certain types of epilepsy. The present study examined the presence of adverse or beneficial cognitive or functional adaptive effects associated with CBD in the treatment of children, adolescents, and teenagers with treatment-resistant epilepsy (TRE) as part of an ongoing prospective, open-label safety study. Participants (N = 38) between the age of 3 and 19 years with TRE were enrolled in an open-label study of a pharmaceutical formulation of CBD (Epidiolex®; GW Research Ltd.) as an add-on treatment. In addition to baseline physical, neurological, and laboratory testing, cognitive assessment was completed prior to initiating CBD and after one year, both using the NIH Toolbox Cognition Battery (NIHTB-CB). Many participants were unable to complete the NIHTB-CB because of the magnitude of their cognitive impairment (n = 24), and in these cases, the participant's caregiver was asked to complete the Adaptive Behavior Assessment System – Second Edition (ABAS-II) as a measure of functional adaptive skills. There were no statistically significant changes in cognitive function, as measured by the NIHTB-CB, in those participants who were able to complete such testing, but there was a nonsignificant trend toward improvement in some cognitive domains. For participants who were unable to complete formal standardized cognitive testing because of the magnitude of their cognitive impairment, their functional adaptive skills, as measured by the ABAS-II, were unchanged after a one-year trial of CBD. Our findings suggest that CBD, as an add-on drug for TRE in a pediatric sample, does not appear to cause adverse effects (AEs) involving cognition or adaptive function over one year of treatment. •Cannabidiol (CBD) was cognitively and functionally well tolerated after one year.•There were no significant changes in cognitive test scores or functional adaptive scores after one year.•Neither CBD dose after one year nor seizure control were associated with scores on the cognitive and functional measures.