Autonomous power systems, such as those existing in small and medium-size islands, typically consist of medium voltage networks fed by conventional power stations and present characteristics not typical for large interconnected grids. In this paper, results are presented and analyzed from a power-quality measurement campaign, conducted in a diesel-powered island system with significant wind penetration. Emphasis is placed on the voltage flicker, which was found quite increased under all operating conditions. It is shown that the diesel engines of the conventional power station are the main source of flicker in the system, due to the pulsed variation of their torque. The operation of the wind farms, on the other hand, does not result in any notable deterioration of the voltage quality level in the system.