  We compare the mass functions of young star clusters (ages $\leq 10$ Myr) and
giant molecular clouds (GMCs) in six galaxies that cover a large range in mass,
metallicity, and star formation rate (LMC, M83, M51, NGC 3627, the Antennae,
and NGC 3256). We perform maximum-likelihood fits of the Schechter function,
$\psi(M) = dN/dM \propto M^{\beta} \exp(-M/M_*)$, to both populations. We find
that most of the GMC and cluster mass functions in our sample are consistent
with a pure power-law distribution ($M_* \rightarrow \infty$). M51 is the only
galaxy that shows some evidence for an upper cutoff ($M_*$) in both
populations. Therefore, physical upper mass cutoffs in populations of both GMCs
and clusters may be the exception rather than the rule. When we perform
power-law fits, we find a range of indices $\beta_{\rm PL}=-2.3\pm0.3$ for our
GMC sample and $\beta_{\rm PL}=-2.0\pm0.3$ for the cluster sample. This result,
that $\beta_{\rm Clusters} \approx \beta_{\rm GMC} \approx -2$, is consistent
with theoretical predictions for cluster formation and suggests that the
star-formation efficiency is largely independent of mass in the GMCs.
