We have previously shown that 5-arylaminouracil derivatives can inhibit HIV-1, herpesviruses, mycobacteria, and other pathogens through various mechanisms. The purpose of this study was to evaluate the potential of 5-arylaminouracils and their derivatives against leukemia, neuroblastoma, and glial brain tumors. 5-Aminouracils with various substituents and their 5'-norcabocyclic and ribo derivatives were screened for cytotoxicity against two neuroblastoma cell lines (SH-SY5Y and IMR-32), K-562 lymphoblastic cells, HL-60 promyeoloblastic cells, and low-passage variants of well-differentiated glioblastoma multiforme (GBM5522 and GBM6138). Cytotoxicity assessment by the standard MTT test showed that most of the compounds lack significant toxicity towards the above cells. However, 5-(4-isopropylphenylamine)uracil and 5-(4-tert-butylphenylamine)uracil exhibited a dose-dependent toxic effect towards the GBM6138 cell line with half-maximal inhibitory concentrations (IC₅₀) of 9 and 2.3 μМ, respectively. Antitumor activity was for the first time demonstrated for compounds of this type and can serve as a starting point for further research.