Structural parameters, derived from X-ray crystallographic data, have been compiled for amino acid and linear peptide derivatives which contain the N-terminal tert-butoxycarbonyl (Boc) group or its next higher homolog, the tert-amyloxycarbonyl group. The comparison of the geometry of the urethane group in Boc-derivatives with that of the peptide group shows small differences in bond angles about the trigonal carbon, because of altered interactions when a CαH group of a peptide unit is replaced by an ester oxygen. In contrast to the strong preference of the peptide bond for the trans form (except when it precedes proline), the urethane amide bond adopts both the cis and trans conformations in crystals. The cis urethane conformation is preferred in crystals of compounds with a tertiary nitrogen (such as Boc-Pro) or in structures stabilized by strong intermolecular interactions. Conformational energy computations on Boc-amino acid N'-methylamides indicate that the trans and cis conformations of the urethane amide bond have nearly equal energies (even for amino acids other than proline), in contrast to the peptide bond, for which the trans conformation has a much lower energy. The computed increase of the cis content in Boc-amino acid derivatives (as compared with the corresponding N-acetyl derivatives) is consistent with the observed distributions of conformations in crystal structures and with n.m.r. studies in solution. Usually, the substitution of a Boc for an N-acetyl end group does not alter the conformational preferences (as indicated by φ, Ψ values and relative energies) of the amino acid residue which follows the end group when the amide bond is trans. Particular conformations, however, can be stabilized by strong attractive interactions between some side chains (e.g. that of phenylalanine) and the bulky Boc end group.