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Abstract: Experimental results on the behaviour of a freely vibrating circular cylinder in laminar and turbulent flows are presented. Wind tunnel tests have been performed by varying the cylinder roughness and the mass-damping parameter, . The effects of surface roughness and flow turbulence have been considered in term of an effective Reynolds number, . In this way, it has been possible to test the sample before and after the beginning of the cylinder boundary layer transition. It has been evidenced that is not the sole parameter influencing the response during lock-in; Reynolds number also plays an important role. If is fixed, the responses decrease as increases. Moreover, it has been observed that the cylinder may experience vortex-induced vibrations even in the critical regime characterized by a cessation of coherent vortex shedding in steady configuration. The effects of and on the critical velocity (the velocity at which the maximum amplitude of response is approached), on the wideness of the lock-in range, and on the wake correlation length are also discussed. [Copyright &y& Elsevier]