In its early days, Amsterdam had dozens of convents and monastries and was a bit of a centre of religious fervour. The miracle of Amsterdam only helped boost this religious tourist trade.
According to legend, a priest was summoned to the bed of a dying man. After receiving the Last Sacrement, the man threw up and the vomit was put on the fire. The following morning the host was found to be undamaged by the flames. Soon a chapel was built on the site, and the road to this chapel became known as the Holy Way (Heilige Weg). A street still bears this name.
From then on a procession of pilgrims, inspired by a series of miracles associated with the chest in which the untouched wafer was placed, followed the Holy Way, although the custom died out when Amsterdam turned officially protestant in 1578. Since the turn of this century the procession has been revived and some 10,000 people take part in it every spring.
The chest which allegedly contained the vomit, but probably stored documents, is on view at the Amsterdamse Historisch Museum.