As part of a GARTEUR project on "The Initial Flaw Concept and Short Crack Growth" a round robin investigation has been performed. The aim of the investigation was to determine equivalent initial flaw sizes (EIFS) for two sets of fatigue life data using different methods. The fatigue life data as well as the necessary material data were provided by BAe Airbus, UK, The general method applied to evaluate the EIFS was crack growth back extrapolation from the critical crack size to the EIFS corresponding to time zero. Four different types of back extrapolations were used. The Paris´ law with two different sets of parameters, the inverse hyperbolic tangent relation combined with the US Airforce initial fatigue quality model and the Paris´ law combined with the strip yield model. The EIFS obtained Showed an influence of the stress level, independently of the method used. Also, the EIFS became very small especially at the lower stress levels. Fitting Weibull compatible or log-normal distributions to the EIFS distributions was possible with rather high accuracy. However, using these distributions in Monte Carlo simulations showed that the experimental fatigue lives could not be recovered.