[en] At the ATLAS experiment events with jets and missing transverse momentum show a large discovery potential for many SUSY models. In these scenarios decay chains of gluinos and squarks lead to the production of two lightest supersymmetric particles (LSPs) which pass through the detector without interactions. Many Standardmodel processes produce the same signature and form background to the SUSY signal. One of the dominant irreducible backgrounds with large missing transverse energy is Z→ νν + jets. The Z→ νν background can be estimated by measuring the same process but with the difference the Z-boson decays into leptons. We present a procedure to estimate this background and its systematic error using a combination of data and simulated events. The systematic studies were performed using simulated event samples at a centre-of-mass energy √(s) = 10 TeV, first results for 7 TeV are also presented. Due to high cross sections and clean final states with muons and electrons, the inclusive production of Z events can already be studied with the first 100 pb^-1 of data collected with the ATLAS detector. We have also begun to investigate background determination using first LHC data.