A remarkable inequality, with utterly explicit constants, established by Erdélyi, Magnus, and Nevai, states that for > − 1 2 , the orthonormal Jacobi polynomials P[Erdélyi et al., Generalized Jacobi weights, Christoffel functions, and Jacobi polynomials, SIAM J. Math. Anal. 25 (1994), 602-614]. They conjectured that the real order of the maximum is O( 1/2 ).Here we will make half a way towards this conjecture by proving a new inequality which improves their result by a factor of order ( 1 + 1 k ) −1/3 . We also confirm the conjecture, even in a stronger form, in some limiting cases.