A blade for a gas turbine includes an airfoil extending in radial direction of the turbine or longitudinal direction of the blade, respectively, between a platform and a blade tip. The airfoil is bordered across the airfoil by a leading edge and a trailing edge and has a suction side and a pressure side. At the trailing edge a first cooling passage runs parallel to the trailing edge from the platform to the blade tip in the interior of the airfoil. The cooling passage is supplied with a cooling air flow from the platform side, and from which cooling air is discharged through a plurality of cooling holes arranged all over the blade. For such a blade the cooling is optimized by providing a first cooling passage, the passage area of which is tapered in radial direction by between 35% and 59%.