The migrant laborers from Myanmar, Lao PDR and Cambodia are the largest groups of foreign workers in Thailand. Effective policy making and planning related to migrant laborers and their families is impeded by the fact that they are a mix of both legal and illegal migrants which makes it difficult to determine the precise number in each group.This study attempts to provide more accurate size estimates of these groups than has been available so far. Our estimates of the populations rely on both primary and secondary sources of data. Data from a sample of 21 Census Enumeration Areas (EA) in Bangkok were augmented by the registration of foreign migrant laborers. The size estimates based on these data indicate that the total number of migrant laborers and their dependents from these three countries residing in Bangkok is in a range of 464,943 to 681,867. It is recommended that comparable surveys be conducted in other provinces to provide a more accurate national estimate of the total migrant labor populations, including those from these three neighboring countries. Data on the migration patterns, as well as documentation of the speed and patterns of movement while in Thailand would further refine these estimates.