This paper deals with the reliability analysis of gas-insulated transmission lines (GIL). Two scheme alternatives of a GIL system have been investigated from a reliability standpoint. These configurations are of basic interest since they recognize how reliability performance may be improved by introducing redundancies at either the system level or at the unit level. Of course, reliability can only be increased with higher investment costs: these can be justified when a high level of transmission-line reliability is required. For the connection of a new powerplant to a node of the existing electric grid, the cost of undelivered energy can play a key role. Therefore, in the context of an electric energy market where the transmission grid ensures the efficient matching of demand and supply, a tool for evaluating the availability of a transmission line may be extremely important.