  Ever since their discovery, Infrared dark clouds (IRDCs) are generally
considered to be the sites just at the onset of high-mass (HM) star formation.
In recent years, it has been realized that not all IRDCs harbour HM Young
Stellar Objects (YSOs). Only those IRDCs satisfying a certain mass-size
criterion, or equivalently above a certain threshold density, are found to
contain HMYSOs. In all cases, IRDCs provide ideal conditions for the formation
of stellar clusters. In this paper, we study the massive stellar content of
IRDCs to re-address the relation between IRDCs and HM star formation. For this
purpose, we have identified all IRDCs associated to a sample of 12 Galactic
molecular clouds (MCs). The selected MCs have been the target of a systematic
search for YSOs in an earlier study. The catalogued positions of YSOs have been
used to search all YSOs embedded in each identified IRDC. In total, we have
found 834 YSOs in 128 IRDCs. The sample of IRDCs have mean surface densities of
319 Mo/pc2, mean mass of 1062 Mo, and a mass function power-law slope -1.8,
which are similar to the corresponding properties for the full sample of IRDCs
and resulting physical properties in previous studies. We find that all those
IRDCs containing at least one intermediate to high-mass young star satisfy the
often-used mass-size criterion for forming HM stars. However, not all IRDCs
satisfying the mass-size criterion contain HM stars. We find that the often
used mass-size criterion corresponds to 35% probability of an IRDC forming a
massive star. Twenty five (20%) of the IRDCs are potential sites of stellar
clusters of mass more than 100 Mo.
