The aim of this paper is to investigate the mixed convection and flow reversal in a vertical concentric annular duct, with reference to laminar and fully developed flow of a Bingham plastic. The boundary walls are supposed to be isothermal, with different temperatures. The dimensionless mass, momentum and energy balance equations are written together with the viscous stress constitutive equation for a Bingham plastic and then solved analytically. The dimensionless velocity, temperature and viscous stress fields, as well as the dimensionless pressure drop parameter, are obtained. The limiting cases of forced convection and of Newtonian fluid are considered.