The corrosion of steel reinforcing bars in concrete is an electrochemical phenomenon in which the oxygen reduction is the controlling step in the overall corrosion process, no matter whether the steel bars are in the passive or active state. The concern is whether the measurement of the limiting current density of the oxygen reduction is a useful tool to predict the corrosion current density and thus the corrosion rate. If the corrosion current density is determined by the limiting current density of the cathodic reaction, both values should be similar. In this work a compilation of more than 1700 data on corrosion current density vs. oxygen limiting current density was analyzed, including data measured on structures exposed to outdoor environments and data measured on specimens exposed to laboratory-controlled conditions. It was observed that in all cases the corrosion current densities measured are lower than the oxygen limiting current densities. This implies that the measurement of the oxygen limiting current density does not allow to assess the corrosion current density, and thus the only way to know the rate at which a concrete structure is corroding is directly by measuring the corrosion current density.