Nowadays, an important process applied in the design of novel composite materials and drug delivery fields is the carboxylation of carbon nanotubes. In this work, we study the interaction of the anticancer drug hydroxyurea with carboxyl-functionalized zigzag carbon nanotubes (CNTs) by employing the method of the density functional theory (DFT) at B3LYP and CAM-B3LYP levels in gas and solvent phases. The results show that all complexes are energetically favorable, especially in the aqueous phase. The enthalpy energy values are negative in all cases, which indicate their exothermic adsorption nature. The presence of COOH groups would create enough free space on the nanotube surface for the adsorption between interacting atoms. Thus, these can increase the activity of CNTs. Data indicates that adsorption is dependent on the carboxyl sites of the nanotube as well as on the sites of the drug. Furthermore, the hydrogen-bonding interactions between drug and COOH-CNTs play an important role for the different kinds of adsorption observed. (C) 2017 Elsevier B.V. All rights reserved.