The microscopic inter-face asymmetry of (001)-grown semiconductor heterostructures that gives rise to heavy-light hole coupling even at zero in-plane wave vector k(parallel to), modifies also the subband dispersion of confined electrons. Starting from a multiband envelope formulation we apply matrix perturbation theory to derive explicit expressions caused by this interface asymmetry, which in the 2 X 2 conduction band Hamiltonian appear as a warping and a spin-splitting term. The warping term results in an inequivalence of the dispersion along [110] and [1 (1) over bar0] as required by the microscopic C-2v symmetry, while the spin-splitting term has the same structure as the corresponding term derived from the zinc-blende bulk inversion asymmetry. Implications with respect to spin-relaxation will be discussed. (C) 2002 Elsevier Science Ltd. All rights reserved.