The stability of baroclinic circular vortices in an incompressible and inviscid fluid for axially symmetric disturbances is discussed, and the well-known necessary condition for instability is shown to be also a sufficient one in the absence of friction. The customary approach to stability problems by means of the eigen-solution with a separate, time-dependent factor is not generally applicable because eigen-solutions do not necessarily exist in this problem. Instead, the perturbation equation for meridional motion is considered as an initial value problem, and the stability of a given circular vortex is determined from consideration of the total kinetic energy of meridional motion. That is, the vortex is considered stable if the total kinetic energy of the perturbation is always bounded for any initial disturbance; and the vortex is considered unstable if there exists at least one set of initial data for which the total kinetic energy of the perturbation would increase with time beyond limit. Wi... Abstract The stability of baroclinic circular vortices in an incompressible and inviscid fluid for axially symmetric disturbances is discussed, and the well-known necessary condition for instability is shown to be also a sufficient one in the absence of friction. The customary approach to stability problems by means of the eigen-solution with a separate, time-dependent factor is not generally applicable because eigen-solutions do not necessarily exist in this problem. Instead, the perturbation equation for meridional motion is considered as an initial value problem, and the stability of a given circular vortex is determined from consideration of the total kinetic energy of meridional motion. That is, the vortex is considered stable if the total kinetic energy of the perturbation is always bounded for any initial disturbance; and the vortex is considered unstable if there exists at least one set of initial data for which the total kinetic energy of the perturbation would increase with time beyond limit. Wi...