This article describes the methodology for projecting business-as-usual GHG trajectory developed in technical work for South Africa's Long-Term Mitigation Scenarios (LTMSs), in particular the 'Growth without Constraints' (GWCs) scenario. Technically rigorous projections are important as developing countries define their commitment to act on mitigation relative to business-as-usual (BAU). The key drivers for the GWC scenario include GDP (both growth rate and composition), population, discount rate and technological change. GDP emerged as an important driver in the research for LTMS and further analysis. If South Africa's economy grows without constraints over the next few decades. GHG emissions will continue to escalate, multiplying more than four-fold by mid-century. There is little gain in energy efficiency, and emissions continue to be dominated by energy use and supply, the latter remaining coal-based in GWC. We analyse the projections (not predictions) in relation to various measures. The LTMS GWC scenario is compared to other projections, nationally and internationally. A broadly comparable projection is being used at national level, for electricity planning. When compared to projections from international models, we find that the assumptions about GDP growth rates are a key factor, and suggest that comparisons of global data-sets against national analyses is important. (C) 2011 Elsevier Ltd. All rights reserved.