Fully nonlinear wave interactions with a three-dimensional body in the presence of steady uniform currents are studied using a Numerical Wave Tank (NWT). The fully nonlinear NWT simulations are compared with perturbation-based time-domain solutions. The three-dimensional NWT uses an indirect Desingularized Boundary Integral Equation Method (DBIEM) and a Mixed Eulerian-Lagrangian (MEL) time marching scheme. The Laplace equation is solved at each time step and the fully nonlinear free surface boundary conditions are integrated with time to update its position and boundary values. A regridding algorithm is devised to eliminate the possible saw-tooth instabilities. The incident waves are generated by a piston-type wavemaker and the current is introduced in the whole fluid domain at the start of simulations. The outgoing waves are dissipated inside a damping zone by using spatially varying artificial damping on the free surface. Computations are performed for the nonlinear diffractions of steep monochromatic waves by a truncated vertical cylinder in the presence of uniform coplanar or adverse currents. The NWT simulations are also compared favorably with the experimental results of Mercier and Niedzwecki  and Moe . (C) 1998 Elsevier Science Ltd. All rights reserved.