Let ξ = (ξ i , 1 ≤ i ≤ n) and η = (η i , 1 ≤ i ≤ n) be standard normal random variables with covariance matrices R 1 = (r 1 ij ) and R 0 = (r 0 ij ), respectively. Slepian's lemmais at least 1. In this paper an upper bound is given. The usefulness of the upper bound is justified with three concrete applications: (i) the new law of the iterated logarithm of Erdős and Révész, (ii) the probability that a random polynomial does not have a real zero and (iii) the random pursuit problem for fractional Brownian particles. In particular, a conjecture of Kesten (1992) on the random pursuit problem for Brownian particles is confirmed, which leads to estimates of principal eigenvalues.