1. A method for monitoring the condition of an individual machine belonging to a selected class of machines, wherein operational data significant for that condition is compared with a reference data according to a predetermined criterion, and wherein an output signal indicating the result of the comparison is formed, characterised in that the method comprises the steps of: creating a first set of operational data for the selected class of machines on the basis of data stored in a reference database, creating a second set of operational data for the individual machine based on measurements performed on that machine, forming a first reference data in dependence on values of at least a selection of said first set of operational data, forming a second reference data in dependence on values of said second set of operational data, comparing said second reference data with said first reference data in accordance with the first predetermined criterion, and forming the output signal in dependence on the result of said comparison.