We study the evolution of scalar curvature perturbations in a brane-world inflationmodel in a 5D anti-de Sitter space–time. The inflaton perturbations are confinedto a 4D brane but they are coupled to the 5D bulk metric perturbations. Wenumerically solve full coupled equations for the inflaton perturbations and the5D metric perturbations using the Hawkins–Lidsey inflationary model. At aninitial time, we assume that the bulk is unperturbed. We find that the inflatonperturbations at high energies are strongly coupled to the bulk metric perturbationseven on subhorizon scales, leading to the suppression of the amplitude of thecomoving curvature perturbations at a horizon crossing. This indicates that thelinear perturbations of the inflaton field do not obey the usual 4D Klein–Gordonequation due to the coupling to 5D gravitational fields on small scales, and it isnecessary to quantize the coupled brane–bulk system in a consistent way in order tocalculate the spectrum of the scalar perturbations in a brane-world inflation.