Hydraulic conductivities of Boom Clay measured through various testing techniques in the laboratory, exhibit similar K values in the order of 10− 12 m/s. Spatial analysis at the Mol site reveals a typical profile with a very homogenous 61-m thick central part, i.e. the so-called Putte and Terhagen Members, which is also the least permeable part of the Boom Clay. The geometric mean of the vertical (Kv) and horizontal (Kh) hydraulic conductivities for the Putte and Terhagen Members at the Mol site are 1.7 × 10− 12 and 4.4 × 10− 12 m/s, respectively, with a vertical anisotropy Kh/Kv of about 2.5. A regional analysis of vertical K variability of the Boom Clay in the northeast of Belgium based on test results from five boreholes shows an increase in hydraulic conductivity from the east towards the west. Statistical analyses indicate that the effect of the samples' stratigraphic position on hydraulic conductivity is strongly related to different grain-size characteristics. However, a general K–grain-size model does not explain the geographical differences in K values satisfactorily. The regional variation in K could be attributed in part to porosity, which in turn is related to the burial depth of the clay.