For more than 40 years something special has been going on in Kalundborg: manufacturing companies have worked together with the municipality and the local utility in order to save resources. An industrial ecosystem has been developed, named Kalundborg Symbiosis and organized as a private association. No doubt, the industrial symbiosis created local growth and an increased competitiveness in the companies over the years, but it was difficult to answer exactly how big the environmental and economic savings were, since symbiosis was just “usual business”. Therefore Kalundborg Symbiosis in partnership with Symbiosis Center Denmark has commissioned an analysis of the Symbiosis’ effect on CO₂ emissions and resource savings. This paper also describes an economic analysis of the savings generated by Kalundborg Symbiosis compared with a hypothetical situation where it did not exist.