Cognitive radio has been identified as a promising solution to the so-called spectrum scarcity problem. The basic idea of this paradigm is to allow unlicensed users to access in an opportunistic and non-interfering manner some licensed bands temporarily unoccupied by licensed users. Cognitive radio is expected to dramatically improve the efficiency of spectrum usage. However, before this paradigm can turn into reality, a full understanding of the spectrum dynamic usage in real wireless communications systems is required. Current spectrum utilization has already been evaluated in some measurement campaigns all over the world. However, to the best of the authors’ knowledge, no previous study has assessed the spectrum occupancy and the feasibility of the cognitive radio technology in different allocated spectrum bands under Spanish regulations. In this context, this paper presents a pioneer spectrum occupancy measurement in Spain conducted in the frequency range from 75 MHz to 7075 MHz in an outdoor environment in urban Barcelona. The measurements are analyzed and compared to the official spectrum regulations. The obtained results demonstrate the existence of a significant amount of spectrum available for the deployment of future cognitive radio networks.