A transcript survives of the oath sworn in 1465 by the lay sacristan of the collegiate church of St Mary at Warwick on the occasion of his taking office. His duties are spelled out in detail, and include the striking requirement that he spend each night in the sacristy for the better security of the treasures. This paper prints the oath and aims to place it in its institutional context. The medieval sacristy at Warwick survives and details of the oath illuminate details of the architecture. Similar first-floor vestries are known elsewhere, and the suggestion is made that some other churches might also have had inhabited sacristies.