A method for forming a sheet made of an aluminium alloy into a component of complex shape, particularly a motor-vehicle component, such as an outer panel or an inner frame of a bonnet or a door of a motor-vehicle, provides for blow-forming of the sheet, with the aid of pressurized gas, within a mould. The alloy constituting the sheet does not have superplasticity features and the sheet and/or the mould are heated to a temperature in the order of 400°-450° C. for 5XXX series alloys and 450°-500° C. and over for 6XXX and 7XXX series alloys. The maximum pressure reached by the forming gas is in the order of 20-30 bars and the time required for forming the sheet is between 40 and 150 seconds and therefore is consistent with production rates in the automotive field.