Comparison between the two molecular mass estimates: masses deduced from sub-millimeter dust emission and virial masses. The mass deduced from the LABOCA emission in the SMC are shown for a dust temperature as estimated from the SED fitting with β = 2 (T_dust ~ 12 K, black diamonds) and for a dust temperature of 19 K as an upper limit (green rhombs). Galactic GMCs studied with the same method are shown for comparison (blue stars). In the SMC, the molecular mass deduced from the sub-millimeter are systematically larger than the one deduced from the virial theorem applied to CO data, even with an upper limit to the dust temperature, in contrast to what is seen for equivalent Galactic clouds.