The spatial variation of the hydraulic conductivity of a complex confining layer in the Netherlands is modeled at four different scales: the scale of sediment cores (10-1-100 m), the scale of the numerical model blocks (101-102 m), the local scale (102-103 m), and the regional scale (103-105 m). Two stochastic methods are combined: (1) conditional (multiple indicator) simulation of texture classes to obtain three-dimensional images of the lithology of the confining layer from borehole descriptions and (2) the statistical upscaling procedure of Indelman and Dagan <1993a,b> to derive the first- and second-order statistics of hydraulic conductivities at the scale of the numerical model blocks. When the upscaling of the core-scale conductivities is ignored, the local- and regional-scale groundwater flow through the confining layer is predicted wrongly. ¿ American Geophysical Union 1996