In this paper we compare estimates of immigrants' labour supply assimilation profiles using the Current Population Survey Annual Demographic Files (March ADS) and the Current Population Survey Outgoing Rotation Groups (ORGs). We use a measure that is seemingly consistent across both surveys: usual weekly hours of work in the main job. This measure is not only important as it measures the intensive margin of labour supply, but it is also used to estimate hourly earnings in both surveys. Our results indicate that the two surveys produce dramatically different estimates of the change in average hours of work as immigrants' years in the United States increase: estimates from the March ADS predict much steeper hour's assimilation profiles than do estimates obtained from the ORGs. These differences persist if we include controls for demographic characteristics, and only disappear once we control for occupation heterogeneity. We argue that these differences stem from two separate factors that differentiate the data. First, the ADS and ORG frame the usual “hours worked” question differently. Also, differences in the timing of the surveys may produce seasonality effects that differentially affect the composition of recent and earlier migrants, thereby changing assimilation profiles.