•Anisotropic fatigue properties were observed in SLM AlSi10Mg in different build directions.•X-CT was adopted to characterize the size, shape, and location of 3D defects.•The anisotropic fatigue resistance is largely controlled by the spatial anisotropy of defects.•Fatigue strength assessment considering defect distribution was proposed via K-T diagram. Metallurgical defects have a critical influence on the anisotropic fatigue resistance of additively manufactured parts under cyclic loading. Here X-ray computed tomography (CT) has been used to characterise the defect population for laser powder bed fusion processed AlSi10Mg alloy and correlated with the tensile and high cycle fatigue (HCF) properties of specimens loaded both parallel and perpendicular to build direction. Despite similar tensile strengths, those tested perpendicular to the build direction exhibit a higher elongation and a higher fatigue strength (114 MPa) than those tested parallel to it (45 MPa). The near surface defects preferentially act as the fatigue crack initiation site for almost all the tested HCF specimens. The large oblate (pancake-shaped) defects were found to orient primarily within the build plane giving a larger projected area within this plane leading to a highly anisotropic fatigue strength. Extreme value statistics were used to predict the likely defect population in the critical near surface region of fatigue samples based on X-ray CT measurements. Finally, a fatigue performance assessment diagram considering these extreme value defects was established using the Kitagawa-Takahashi diagram.