We point out that for a large class of universes, holography implies that the most probable value for the cosmological constant is zero. In four space-time dimensions, the probability distribution takes the Baum-Hawking form, dP ϳ exp͑cM 2 p ͞L͒dL. The cosmological constant problem has a twofold meaning: It is a problem of fundamental physics, because the value of the cosmological constant L is tied to vacuum energy density. On the other hand, the cosmological constant tells us something about the large scale behavior of the universe, since a small cosmological constant implies that the observable universe is big and (nearly) flat. The problem is that there is an enormous discrepancy between the value of the vacuum energy density as predicted by quantum field theory of the standard-model degrees of freedom and the cosmologically observed value of L [2]. This discrepancy occurs already at very low-energy scales, of the order of eV, and clearly represents the most flagrant naturalness problem in today's physics.Thus, the cosmological constant relates the properties of the microscopic physics of the vacuum to the long-distance physics on cosmic scales. [This general philosophy was stressed in the wormhole approach to the cosmological constant problem (see [3] for the original references and [4,5] for a critique of this approach).] Therefore, the observable smallness of the cosmological constant should tell us something fundamental about the underlying microscopic theory of nature.In this note we study implications of holography [6,7], taken as a fundamental property of the microscopic theory of quantum gravity, for the cosmological constant problem. Assuming that the cosmological constant is a random variable, and that holographic entropy can be given a Boltzmannian interpretation, we point out that the most probable value of the cosmological constant in a holographic theory is zero, in ensembles of universes with finite-area holographic screens.It is not the aim of this paper to discuss the microscopic origin of any of these assumptions. In particular, the microscopic origin of holography and the randomness of the cosmological constant are clearly difficult problems whose solutions we do not claim to have understood. It is our goal to show that once these assumptions are satisfied, a simple and robust phenomenological argument implies a naturally small cosmological constant.