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We apply the holonomic gradient method introduced by Nakayama et al. [23] to the evaluation of the exact distribution function of the largest root of a Wishart matrix, which involves a hypergeometric function 1 F 1 of a matrix argument. Numerical evaluation of the hypergeometric function has been one of the longstanding problems in multivariate distribution theory. The holonomic gradient method offers a totally new approach, which is complementary to the infinite series expansion around the origin in terms of zonal polynomials. It allows us to move away from the origin by the use of partial differential equations satisfied by the hypergeometric function. From numerical viewpoint we show that the method works well up to dimension 10. From theoretical viewpoint the method offers many challenging problems both to statistics and D-module theory., where 0 ≤ n 1 , . . . , n m ≤ 2 and at most one of n 1 , . . . , n m is two. Denote [m] = {1, . . . , m}. For a subset J ⊂ [m] denote

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