The history of Poznah’s biggest eucharistic shrine, named after Corpus Christi, goes back to the end of the 14th century and a miracle recorded only very imprecisely in the earliest sources. More information can be found in documents from the second half of the 15th century. They allege that in an incident which occurred in 1399 the Host was desecrated by the Jews. That motif, now believed to be secondary, was greatly expanded during the Counterreformation. Since the late 16th century the origins of the shrine were related to the story of the Miracle of Three Hosts. The dynamic growth of the eucharist cult in Poznan attracted the attention of King Władysław Jagiełło. He came up with the idea of founding a church for the Carmelite convent which was being set up in the place associated with the miracle. King Władysław Jagiełło saw the shrine as an acknowledgment and a sign of God’s providence looking after his lands and possessions. The foundations of the church and the choir had been completed by 1434, the year he died. The nave was built in stages during the second half of the 15th century. At first the whole was conceived as a hall church; however, as a result of some disaster that plan was abandoned in favour of a pseudo-hall design. The second phase saw the construction of massive pillars separating the nave from the aisles. In the third phase, which may have extended beyond 1500, the walls were topped up with roofs and ceilings. The plan of the church, and especially the nearly quadrangular outline of the nave, resembles the plans of a number of 14th-century Cracow churches, eg. St Mary’s as well as the churches of the religious orders (the Dominicans, the Augustinians and the Regular Canons). It seems that the skills and expertise involved in the construction of the Cracow churches were recruited for the first stage of the construction of the Corpus Christi church in Poznań. In the following phase the work was taken over by a local workshop, which may have had its roots in Warmia. The name of the master builder Wojciech Łobżeński, which is preserved in the sources, is to be linked with that stage of the construction work. The ground on which the church was being built was swampy and exposed to regular flooding. This may have been the reason for introducing a number of alterations to the plans. The choir was made shorter than in the Cracow churches; the hall structure was replaced with the pseudo-hall system; the number of pillars between the nave and the aisles was increased. A scrutiny of the eastern arcade on the southern side suggests that there used to be a chapel in that place. It may have been the original shrine of the miraculous Host, later (ie. in the 16th century) removed to a new chapel on the northern side of the presbytery.