Family reconstitutions have been undertaken only rarely in urban settings due to the high mobility of historical urban populations, in both life and death. Recently Gill Newton has outlined a methodology for the reconstitution of urban populations and we applied a modified version of this method to the large Westminster parish of St. Martin in the Fields between 1752 and 1812, a period that posed particular difficulties for family reconstitution because of the rapid lengthening of the interval between birth and baptism. The extraordinary richness of the records for St. Martin in the Fields made it possible to investigate burial and baptismal practices in great detail, and the extent and impact of residential mobility. We found that short-range, inter-parochial movement was so frequent that it was necessary to confine the reconstitution sample to windows in which families registered events at a single street address. Using birth interval analysis and the frequencies of twin births it was possible to demonstrate that the registration of birth events was fairly complete, but that many infant and child burials were missed. These missing burials probably resulted from the unreported export of corpses for burial in other parishes, a phenomenon for which we had considerable evidence. The limitations of family reconstitution in this highly mobile and heterogeneous urban population is discussed and we demonstrate some checks and corrections that can be used to improve the quality of such reconstitutions.