The low oxygen concentration (<2 ml L-1) at relatively shallow depths (<100 m) in the coastal upwelling zone and in the adjacent oceanic area is a distinct feature of the eastern boundary Humboldt Current System (HCS) off Peru and northern Chile; it affects the distribution of pelagic organisms and is associated with an important denitrification regime in the water column. Nevertheless, little information is available about the spatial and temporal variability in the distribution of the Oxygen Minimum Layer (OML) present in the HCS and the impact of its variability upon the pelagic system and biogeochemical cycles in the region. The present study reviews the oceanographic data obtained for the area off northern Chile (ca. 18 to 24S, out to 370 km), between 1980 and 1997, with the aim of characterizing the depth distribution of the upper boundary of the OML (1 ml L-1 iso-oxyline, representing also the oxycline) and investigating the association of its interannual variability with changes in coastal sea level and in the equatorial and local thermoclines. The depth of the upper boundary of the OML undergoes pronounced deepening during the occurrence of warm ENSO (El Nino Southern Oscillation) events over the whole area of study, and this, in turn, determines a condition of higher oxygen concentrations in the top 100 m layer. These changes follow closely the patterns of interannual variability in coastal sea level and depth of the thermocline in the area of study during the 1980-1997 period. Most of this variability can be accounted for, as expected from previous studies, by remote forcing originating in the equatorial zone of the Pacific Ocean, as evidenced from the significant correlation between the above local parameters and the interannual variability in the depth of the equatorial thermocline. The 1982-83 and 1997-98 El Nino events seemed to have been the most important ones in terms of their effects upon sea level and depth of the thermocline and oxycline off northern Chile. The potential impacts of the interannual changes in the depth of the upper boundary of the OML upon the pelagic system and biogeochemical cycling in the region are discussed.