The Gulf of Naples is a coastal area in the south-eastern Tyrrhenian Sea (Western Mediterranean). Zones of great environmental and touristic value coexist in this area with one of the largest seaports in the Mediterranean Sea, industrial settlements and many other pollution sources. In such an environment, water renewal mechanisms are crucial for maintaining the ecological status of the coastal waters. In this paper, we focus on the water exchange between the interior of the Gulf and the neighbouring open Tyrrhenian Sea. The surface dynamics of the Gulf have been investigated based on measurements carried out with a high-frequency (HF) radar system. The vertical component of the current field has been provided by the Regional Ocean Modelling System (ROMS) model of ocean circulation. We present the results of a one-year-long analysis of data and simulation results relative to the year 2009. Inshore/offshore exchanges were assessed by looking at the zonal component of the surface and subsurface current field across a transect representing a sort of threshold between the interior of the Gulf and the open sea. This also allows for the reconstruction of the short-term origin of waters found inside the Gulf in the different forcing and circulation conditions.