Let denote two independent real Gaussian and matrices with , each constituted by zero mean independent, identically distributed columns with common covariance. The Roy’s largest root criterion, used in multivariate analysis of variance (MANOVA), is based on the statistic of the largest eigenvalue, 717544cf4b0890a810dc60373ea">, of , where and are independent central Wishart matrices. We derive a new expression and efficient recursive formulas for the exact distribution of 717544cf4b0890a810dc60373ea">. The expression can be easily calculated even for large parameters, eliminating the need of pre-calculated tables for the application of the Roy’s test.