AbstractSurface defects and internal discontinuities are inevitable results of the laser beam welding (LBW) process regardless of the material used. Comprehensive understanding of the fatigue degradation caused by these defects is of major concern for the introduction of LBW into manufacturing or repair processes. The present paper focuses on the effect of inherent welding-induced material flaws on the high cycle fatigue behaviour of the laser welded Ti-6Al-4V butt joints. If the surface quality of the welded joint is sufficiently high, the transition of the crack origin from the surface to the subsurface occurs. The mechanisms of internal fatigue crack formation and growth at the early stages were studied. A typical fish-eye pattern of fracture surface was observed in close proximity to the crack origin. The model based on fracture mechanics for durability prediction in the presence of randomly distributed porosity was developed. The link between theory, modelling and experiment was successfully demonstrated.