We have determined the luminosity function of 250 \mu m-selected galaxies detected in the \sim 14 deg ^ { 2 } science demonstration region of the Herschel-ATLAS project out to a redshift of z = 0.5 .
Our findings very clearly show that the luminosity function evolves steadily out to this redshift .
By selecting a sub-group of sources within a fixed luminosity interval where incompleteness effects are minimal , we have measured a smooth increase in the comoving 250 \mu m luminosity density out to z = 0.2 where it is 3.6 ^ { +1.4 } _ { -0.9 } times higher than the local value .