We report magnetoresistance (MR) measurements on Au/Co/Au sandwiches and Au/Co/Au multilayers, deposited on polished glass substrates in ultrahigh vacuum, with Co thickness ranging between 3 and 30 A˚. The experiments were performed at room temperature in the field range 0–0.8 T and at low temperature 1.3–4.2 K in the field range 0–5 T for different field orientations with respect to the film and to the current. For Co thickness below 11 A˚, the experimental data are consistent with an easy axis perpendicular to the film, in agreement with previous ferromagnetic resonance and magnetization measurements. From the hysteretic behavior of the MR, coercive fields of a few 102Oe at room temperature are observed. In Co bilayers, the MR is enhanced with respect to the MR of simple sandwiches and indicates a square hysteresis loop. The abnormally large MR of the ultrathin Co films is attributed to a Co/Au interface resistance related to walls between Co domains with alternating perpendicular magnetization.