In order to determine the response inside an OWC pneumatic chamber situated at the head of a breakwater to incident waves and assess the possible effects of the breakwater geometry, we have developed a three-dimensional numerical model based on the linear theory. In the model the hybrid element method is applied. The thickness of the remote part of the long breakwater is ignored. Numerical results are obtained for a skeletal geometry of the the Foz do Douro breakwater. The high response in the chamber suggests the need for further account of separation losses at the opening. The strategy is outlined.