The FEEDBACK VERTEX SET problem on unweighted, undirected graphs is considered. Improving upon a result by Burrage et al. (Proceedings 2nd International Workshop on Parameterized and Exact Computation, pp. 192-202, 2006), we show that this problem has a kernel with O(k 3 ) vertices, i.e., there is a polynomial time algorithm, that given a graph G and an integer k, finds a graph G with O(k 3 ) vertices and integer k ≤ k, such that G has a feedback vertex set of size at most k, if and only if G has a feedback vertex set of size at most k . Moreover, the algorithm can be made constructive: if the reduced instance G has a feedback vertex set of size k , then we can easily transform a minimum size feedback vertex set of G into a minimum size feedback vertex set of G. This kernelization algorithm can be used as the first step of an FPT algorithm for FEEDBACK VERTEX SET, but also as a preprocessing heuristic for FEEDBACK VERTEX SET.We also show that the related LOOP CUTSET problem also has a kernel of cubic size. The kernelization algorithms are experimentally evaluated, and we report on these experiments.