In this note, a class of nonlinear uncertain systems are considered, and uncertainties in the systems are assumed to be generated by exogenous dynamics. Robust control is designed by employing nonlinear observers to estimate the uncertainties. It is shown that, if a partial knowledge of the exogenous system is available and its known dynamics meet certain conditions or if input channel of the plant has certain properties, global stability and global estimation convergence can be achieved. In the latter case, the results on stability and convergence hold even if exogenous dynamics are completely unknown but bounded by some known function.