Abstract. For any 1 < p < ∞ and any X, Y ∈ R satisfying |X| ≤ Y , we determine the optimal constant Cp(X, Y ) such that the following holds. If F is a holomorphic function on the unit disc satisfying ReThis can be regarded as a reverse version of the classical estimates of Riesz and Essén. The proof rests on the exploitation of certain families of special subharmonic functions on the plane.Mathematics Subject Classification. Primary 31B05; Secondary 30J99.