During the routine hydraulic pressure test of a boiler following modification, failure by leakage from the drum took place and was traced to a region where extensive multiple cracking had occurred. Catastrophic rupture or fragmentation of the vessel fortunately did not take place. Prior to the test, cracking was present already, extending up to 90% of the wall thickness. Analyses of brownish deposit material did not reveal the presence of any substances likely to cause stress-corrosion cracking of a Ni-Cu-Mo low-alloy steel.