The present paper describes the coupling of the 3D computational fluid dynamics (CFD) code CFX with the best estimate thermal-hydraulic code TRACE. Two different coupling schemes - one explicit and the other semi-implicit - have been tested. Verification of the coupled CFX-TRACE code has first been carried out on the basis of a simple test case consisting of a straight pipe filled with liquid subject to a sudden acceleration. As a second validation step, measurements using advanced instrumentation (wire-mesh sensors) have been performed in a simple, specially constructed test facility consisting of two loops connected by a double T-junction. Comparisons of the measurements are made with calculation results obtained using the coupled codes, as well as the individual codes in stand-alone mode, thereby clearly bringing out the effectiveness of the achieved coupling for simulating situations in which three-dimensional mixing phenomena are important. [All rights reserved Elsevier].