The background field method for N = 2 super Yang-Mills theories in harmonic superspace is developed. The ghost structure of the theory is investigated. It is shown that the ghosts include two fermionic real ω-hypermultiplets (Faddeev-Popov ghosts) and one bosonic real ω-hypermultiplet (Nielsen-Kallosh ghost), all in the adjoint representation of the gauge group. The one-loop effective action is analysed in detail and it is found that its structure is determined only by the ghost corrections in the pure super Yang-Mills theory. As applied to the case of N = 4 super Yang-Mills theory, realized in terms of N = 2 superfields, the latter result leads to the remarkable conclusion that the one-loop effective action of the theory does not contain quantum corrections depending on the N = 2 gauge superfield only. We show that the leading low-energy contribution to the one-loop effective action in the N = 2 SU(2) super Yang-Mills theory coincides with Seiberg's perturbative holomorphic effective action. © 1998 Elsevier Science B.V.