[en] The dynamic fluid-structure interaction (FSI) during hydraulic transient is known to be of special importance for flexible or movable pipeline system. Some kinds of FSI effects can be observed however even for relatively rigidly supported pipeline. Such effects, not anticipated by the classic waterhammer theory, were identified during experiments on waterhammer phenomenon conducted at a laboratory rig in the Szewalski Institute of Fluid-Flow Machinery of the Polish Academy of Sciences in Gdansk (IMP PAN). Additional pressure oscillations of higher frequencies observed during experiments were supposed to be the result of dynamic fluid-structure interaction. The problem of hydraulic transient with FSI effect taken into account has been of IMP PAN interest for some time and the four equation model of the phenomenon was applied and implemented at a computer program. A method of characteristics with time marching procedure and a 'wave method' for solving the resulted finite difference equations were used at the algorithm. Selected measured and computed pressure records during the transient are presented in the paper. The analyses of the results allows to conclude that the additional effects observed at experiments were really produced by FSI effect (Poisson coupling). Some discrepancies between experimental and numerical results exist however and the analysis and attempt to explain the causes of them are proposed as well.