From the existing data in the literature it is shown that there is a linear correlation between cloud mass derived from CO observations and the associated luminosity obtained from far-IR observations over a large luminosity range of 10 to the 4th to 10 to the 8th solar luminosities. The mean value of luminosity per unit mass for a giant molecular cloud is 5.6 solar-L/solar-M. The star-forming efficiency of the molecular cloud over its lifetime of 5-10 x 10 to the 7th yr is found to be 0.2-0.3, which yields the present star-forming rate of 6-11 solar masses per year. Furthermore, the integral luminosity distribution is a power-law with an exponent of about -0.5. The correlation between cloud mass and the far-IR luminosity observed for a sample of nuclei of external galaxies corresponds to molecular clouds similar to those in the Galaxy.