Madness is a central theme in "The Fall of the House of Usher." The story explores the psychological decline of the Usher family, particularly through the characters of Roderick and Madeline Usher. Their mental instability is mirrored by the decaying Usher mansion, symbolizing the intertwined fate of the family and their ancestral home. The pervasive atmosphere of gloom and decay contributes to the sense of madness that permeates the narrative, affecting not only the Ushers but also the narrator.

The Usher family's madness is linked to their history of isolation and inbreeding, which has resulted in psychological and physical ailments. Roderick Usher's acute anxiety and hypersensitivity lead to paranoia and hallucinations. He fears the collapse of his mind and body, which is symbolized by the literal collapse of the Usher mansion. The narrator, initially rational, becomes increasingly affected by the oppressive atmosphere, suggesting that the madness may be supernatural in origin. The narrator himself questions the source of his unease, pondering the mysterious power of the house:

What was it--I paused to think--what was it that so unnerved me in the contemplation of the House of Usher?  It was a mystery all insoluble; nor could I grapple with the shadowy fancies that crowded upon me as I pondered.  I was forced to fall back upon the unsatisfactory conclusion, that...there are combinations of very simple natural objects which have the power of thus affecting us, still the analysis of this power lies among considerations beyond our depth.

The setting of the Usher mansion acts as a character itself, embodying the madness of its inhabitants. The house's dilapidation reflects Roderick's mental instability. The narrator describes the house as having an "excessive antiquity," with fungi covering its exterior, symbolizing decay:

Its principal feature seemed to be that of an excessive antiquity. the discoloration of ages had been great. Minute fungi overspread the whole exterior, hanging in a fine tangled web-work from the eaves. Yet all this was apart from any extraordinary dilapidation. No portion of the masonry had fallen; and there appeared to be a wild inconsistency between its still perfect adaptation of parts, and the crumbling condition of the individual stones. In this there was much that reminded me of the specious totality of old woodwork which has rotted for long years in some neglected vault, with no disturbance from the breath of the external air.

Roderick Usher's belief in the sentience of the house and its surroundings further illustrates his madness. He attributes the family's decline to the arrangement of the stones and vegetation, suggesting a malevolent influence:

"in the long undisturbed endurance of this arrangement, and in its reduplication in the still waters of the tarn. Its evidence—the evidence of the sentience—was to be seen, he said,..."

This belief in the house's sentience and its malicious intent underscores the theme of madness, as Roderick's mental state deteriorates alongside the physical structure of the mansion. The ultimate collapse of the house symbolizes the end of the Usher lineage, consumed by the madness that has plagued them.