In 1585 the Duke of Parma conquered Antwerp, which was followed by a blockade of the river Scheldt. A large number of merchants moved to Amsterdam. Since the beginning of the twentieth century a historical debate has been going on about the economic effects of these events. The submitted data, gathered from different historical sources, aim to provide a precise answer to this question. Information from nine different sources (e.g. poortersboeken of the city of Amsterdam, two tax registers, Notarieel Archief and VOC/WIC-archives) has been combined in an attempt to reconstruct the entire merchant community of Amsterdam between 1578 and 1630. This resulted in a data collection on more than 5,000 wholesale traders. Analysis of the data shows that migration to Amsterdam started long before the siege of Antwerp because of old commercial ties between the two cities. Moreover, the role of the newcomers was of moderate importance. In 1609 the immigrants amounted to one third of Amsterdam's merchant community. The majority of them were young men of modest means, seeking an international career in Amsterdam. Since collaboration between merchants from the north and the south dated back to the 1540s, the influx from Antwerp may be considered part of one single merchant community which developed in the Low Countries in the course of the sixteenth century.