An experimental investigation of concomitant Vortex-Induced Vibration (VIV) and top-motion excitation with a long and vertical flexible cylinder is presented. Underwater optical instrumentation allows to directly measure the cartesian coordinates of 43 targets placed along the model. Standard analyses, based on statistical evaluation of local displacement time-histories and on spectral analysis, are carried out. Additionally to the standard approach, modal decomposition is adopted, allowing to obtain modal-amplitude time-histories. It was found that the concomitant VIV and top-motion excitation significantly affects the flexible cylinder response when compared to the pure VIV tests. When the top-motion excitation has frequency twice the first natural frequency in still water (f t: f N, 1= 2: 1), parametric instability of the first vibration mode occurs concomitant to VIV phenomenon. Such instability leads to a marked increase in the oscillation amplitudes for the whole range of towing speed tested.