We consider functionally uncertain systems which can be written in an output feedback form, where the nonlinearities are functions of the output only. The uncertainty is described by a weighted L-2 norm about a nominal system. and an approximate adaptive design is given which ensures output practical stability. The main result requires knowledge of the weighted L-2 uncertainty level. An upper bound on the LQ performance of the output transient and the control input is derived. where the cost penalises the output transient and the control effort on the time interval where the output lies outside the prescribed neighbourhood of zero to which we achieve convergence.