In this paper we compare estimates of immigrants’ labor 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. 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. 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.