Cell loss is an inherent problem of ATM networks. The magnitude of the service degradation caused by cell loss depends on the application and loss distribution. This paper introduces a new performance criterion, called effective loss, which can quantitatively measure this degradation. Effective loss is particularly suitable for block-oriented transmissions, such as file transfer applications, but can also be applied to a broad range of other applications. In this paper the effective loss measure is applied to the study of the effectiveness of bandwidth reservation mechanisms in an ATM multiplexer. Numerical results demonstrate circumstances under which bandwidth reservation improves performance as well as circumstances in which it degrades performance.>