The fatigue performance of orthotropic steel bridge decks depends on the magnitude and number of stress cycles applied, and the deck details. The stress induced in a deck plate will be reduced by the presence of a stiff surfacing material. This paper describes how the likely fatigue lifetime of an orthotropic steel bridge deck with asphalt surfacings could be estimated using probabilistic assessment methodology. In particular, the daily variations in temperature, which affect the asphalt surfacings, are considered. An example of application of the methodology to a realistic bridge is described.