In this paper, we propose for the first time a decentralized adaptive control scheme without requiring a priori information on subsystem high-frequency-gain signs. This is achieved by introducing Nussbaum-type functions in local adaptive controllers, and developing a neu analysis approach which is quite different from the conventional one used in centralized adaptive control involving a Nussbaum-type function. With the help of integrator backstepping, the decentralized control scheme can also be applied to high-relative-degree systems. The results of this paper enlarge the class of uncertain large-scale systems for which decentralized adaptive control can be designed.