  We discuss the relationship between holographic entropy bounds and
gravitating systems. In order to obtain a holographic energy density, we
introduce the Bekenstein-Hawking entropy $S_{\rm BH}$ and its corresponding
energy $E_{\rm BH}$ using the Friedman equation. We show that the holographic
energy bound proposed by Cohen {\it et al} comes from the Bekenstein-Hawking
bound for a weakly gravitating system. Also we find that the holographic energy
density with the future event horizon deriving an accelerating universe could
be given by vacuum fluctuations of the energy density.
