In this paper, we consider U -statistics whose data is a strictly stationary sequence which can be expressed as a functional of an i.i.d. one. We establish a strong law of large numbers, a bounded law of the iterated logarithms and a central limit theorem under a dependence condition. The main ingredients for the proof are an approximation by U -statistics whose data is a functional of i.i.d. random variables and an analogue of the Hoeffding's decomposition for U -statistics of this type.