A steam generator (2) is described in which a combustion chamber has at least one combustion chamber wall formed by a plurality of longitudinal furnace tubes (10) for passage of an evaporatable flow medium, connected together in gas-tight manner by tube webs. As will be familiar different tubes are likely to be subjected to a heat input in use that varies when the combustion chamber is fired. The tube bores of the furnace tubes are larger where subject to higher heat input than where subject to lower heat input. In particular the tube bores of the furnace tubes are generally larger in the middle region of the combustion chamber wall than at the transversely peripheral regions of the combustion chamber wall.