The state of conservation of the 13th-century stained-glass windows in the choir of Bourges cathedral is extremely worrying: the corrosion of the glass in the form of cratering has reached critical proportions. The reduction in its width has made the glass very fragile; the highly adhesive products of corrosion make the scenes illegible. It will not be possible to specify a protection procedure until a cleaning method has been developed that enables the products of deterioration to be eliminated, the latter being resistant to the chemical solutions so far employed. -- AATA