  We present Monte Carlo radiative transfer simulations for spiral galaxies
modelled as a stellar disk and a two-phase clumpy dust distribution. We divide
the volume occupied by the dust into a three-dimensional grid and assign each
cell a clump or smooth medium status. Cell dimension, clump dust mass and
spatial distribution are derived from the observed properties of Giant
Molecular Clouds and molecular gas in the Galaxy. We produce models for several
values of the optical depth and fraction of the interstellar medium residing in
clumps. As a general result, clumpy models are less opaque than the
corresponding homogeneous models. For the adopted parameters, the increase in
the fraction of energy that escapes the disk is moderate, resulting in surface
brightness profiles that are less than one magnitude brighter than those of the
homogeneous models. The effects of clumping are larger for edge-on views of the
disk. This is in contrast with previous preliminary results for clumping in the
literature. We show how differences arise from the different parametrisation
and clump distribution adopted. We also consider models in which a fraction of
the stellar radiation is emitted within the clumps. In this case, galaxies are
less transparent than in the case when only dust is clumped. The opacity can be
even higher than in the homogeneous case, depending on the fraction of embedded
stellar emission. We point out the implications of the results for the
determination of the opacity and dust mass of spiral galaxies.
