Dimerization of calix[4]arene ureidopeptides is demonstrated for the first time. Two calix[4]arenes tetrasubstituted in the upper rim with -NHCONH(L)LeuNHC(8)H(17) (I) and -NHCONH(L)Leu(D)Leu-OMe (2) were prepared and studied by NMR, circular dicroism, and gel permeation chromatography. Compound 2 self-assembles through urea-urea hydrogen bonds, as well as by an additional set of hydrogen bonds provided by the peptide side chains, with participation of the ester carbonyls. The absence of such group in 1 causes the monomer structure to be favored in this case.