The effect of fuel type (liquefied natural gas vs. kerosene) on the optimum cruise altitude for a single-aisle, medium-range transport aircraft is investigated. An automated aircraft synthesis program including a detailed mission analysis module was used to assess the impact of liquefied natural gas (LNG) on fuel burn and equivalent CO2 emissions. Verification studies for four different reference aircraft showed that the mission analysis module was able to predict the fuel weight for various missions with an error between -1.5% and +5.9%. A baseline, kerosene-fuelled, mid-range, single-aisle aircraft was designed which was estimated to have an optimum cruise altitude for minimum fuel burn of 11,400m (37,500ft). It also showed the altitude for minimum equivalent CO2 emissions to be at 12,500m (41,000ft). Changing to LNG, the aircraft showed a maximum reduction in CO2-equivalent emissions of 21% at a cruise altitude of 10,400m (34,100ft). At altitudes above 9,000 meters, the higher H2O emissions rapidly increased the equivalent CO2 emissions. It was therefore concluded that, to a much larger degree than for kerosene-fuelled aircraft, cruise altitude is an important design parameter for LNG-fuelled aircraft if equivalent CO2 emissions are to be minimized.