We examine drivers of cost overruns in Norwegian development projects in the oil and gas sector. The multivariate longitudinal econometric analysis employs a unique and detailed dataset consisting of 79 different projects between 2000 and 2013. Among the significant results, we find that the unexpected change in economic activity has a positive effect on the overruns, there is a considerable positive momentum in the transitional cost overruns, more experienced operators tend to incur less overruns and finally that the size of the investment of the projects has a positive impact on the overruns. Furthermore, we find evidence that current economic activity matters to an extent, but that the pivotal factor is the unexpected change in activity. (C) 2017 Elsevier Ltd. All rights reserved.