The invention relates to a method and system for determining the cardiorespiratory fitness level of a person, with the aid of freely performed exercise in which training: •the physiological intensity of the person is measured periodically during the exercise session (5) •the external work output of the exercise session is measured simultaneously relative to the measured intensity (6), •from each period's measured intensity value and external work output, the representativeness of the values measured is determined (8.1, 8.2), in order to determine the fitness level using the following criteria: ∘the physiological intensity should be stabilized relative to the external work output, ∘the external work output should be within a selected range, ∘the physiological intensity should be greater than a selected lower limit, in which case, a fitness level estimate is defined (8.3) for each accepted period as well as a fitness from several estimates.