We design an adaptive output feedback controller for global asymptotic tracking of a class of nonlinear systems involving unknown parameters and unmeasured states multiplied by output nonlinearities. A modified high-gain observer is introduced, which consists of a series of high-gain K-filters with one dynamic gain and the controller design employs the adaptive backstepping approach. It is proved that by using the proposed method, global stability of the closed loop system can be guaranteed and the output tracking error converges to zero asymptotically. (C) 2012 Elsevier Ltd. All rights reserved.