We construct an effective Hamiltonian for the motion of t 2g highly correlated states in Na x CoO 2 . This three-band model includes the indirect Co–O–Co hopping t and the crystal-field splitting 3D. Calculations in a CoO 6 cluster give the effective parameters t=100meV and 3D=315meV. The Hamiltonian is solved using a generalized slave-boson mean-field approximation. The results show a significant band renormalization. Without any additional hypothesis, the dispersion of the bands near the Fermi energy and Fermi surface topology agree with angle-resolved photoemission experiments, in contrast to predictions using the local-density approximation.