Density functional theory is used to study pore formation in bilayer membranes in amphiphile solutions. The theory is applied to an interaction site model of surfactant molecules composed of two tangent spheres, with effective interactions that mimic the effect of the solvation forces induced by water. The free energy functional of the system exhibits local minima corresponding to planar structures, representing bilayer membranes. Pores in the bilayer are characterized by the rearrangement of the hydrophilic sites along the rim of the pore to shield the hydrophobic sites from solution. Our mean-field approach predicts the formation of stable microholes with a radius comparable to the molecular length. For membranes with a positive surface tension, the energy cost of creating a hole passes through a maximum as a function of the pore's radius, as predicted by classical nucleation theory. For large pores, the actual values of the nucleation barrier and the size of the critical nucleus can be approximated using a modified classical expression based on the surface tension and line tension of the planar bilayer. (C) 2003 American Institute of Physics.