An attempt has been made to study, theoretically, the difference in the fatigue behaviour of non-load-carrying and load-carrying fillet welded cruciform joints. In the former case, the pre-existing flaws produced during the welding process will always propagate from the weld toe. Accurate stress analysis using the boundary element method on the load-carrying fillet welded cruciform joints shows that the weld throat, h, should not be less than 1.414 T, or the weld leg length, w, should not be less than 2.0 T, where T is the main plate thickness, for the cracks to initiate from the weld toe. Furthermore, the values of Mk, the correction factor to take into account the presence of the weld, are found to decrease when the weld size is increased for the load-carrying fillet welded joints. The Paris crack propagation law is then computed numerically to obtain the S/N (signal-to-noise) curves for the two welded joints. The non-load-carrying fillet welded joints give 5 per cent higher fatigue life than the load-carrying ones for an applied stress range of 100 N/mm2.