The paper addresses the global output tracking of a class of uncertain output feedback systems affected by disturbances which are generated from a known exosystem. The system uncertainty is parametrized by a unknown parameter vector, which is absorbed in the parametrization of disturbances. A controller based on the estimation of states and disturbances from the output is designed using backstepping to asymptotically track arbitrary trajectories and to guarantee the boundedness of all other variables. The proposed control algorithm provides an alternative with global tracking to structurally stable regulation of nonlinear output feedback systems. It can also be viewed as a generalization to adaptive control algorithms.