Input-output analysis has been applied to assess macroeconomic impacts of investment expenditures required for introduction of natural gas (NG) into the Greek energy system. The final demand vector was assembled from figures estimated in a prefeasibility study. A 12 x 12 input-output table was used to calculate relative changes in gross domestic product (GDP) for the entire economy, sectoral production and value-added, employment, and wages. We show that construction of the national gas grid will significantly affect all five macroeconomic indicators during a period of eight years. Taking into account direct and indirect impacts, GDP will rise by 2.0% whereas employment and wages will increase by 1.6%. If imports are fully replaced by local produce, GDP will rise by 3.0% whereas employment and wages will increase by 2.4 and 2.3%, respectively. The relative change of production for five specified sectors is greater than 24% during the period 1993-2000.