Let g(x1,..., x(k)) be a symmetric function with k arguments. Let U be a U-statistic based on a random sample of size n with kernel function g. In this paper, the problem of estimating var(U) is considered. Several estimators are compared by computer simulations and we conclude that two estimators, one is constructed as a U-statistic and the other is the bootstrap estimator, give good estimates for many U-statistics.