What mental illness does the narrator in "The Tell-Tale Heart" have?

"How, then, am I mad?"

According to the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition, which is a standard reference manual used to diagnose and classify mental disorders, the unnamed first-person narrator of Edgar Allan Poe's classic short story "The Tell-Tale Heart" may suffer from schizophrenia, a serious mental disorder characterized by severe disturbances in thought, perception, and behavior.

Symptoms of schizophrenia demonstrated by the narrator include delusions, hallucinations, irrational emotional responses, and anosognosia, which is an inability to recognize or accept his own true mental health condition.

In his delusion about the state of his mental health, the narrator tells the reader that "the disease had sharpened my senses—not destroyed—not dulled them" and that "above all was the sense of hearing acute. I heard all things in the heaven and in the earth. I heard many things in hell." He also boasts about his own mental abilities:

You should have seen how wisely I proceeded—with what caution—with what foresight—with what dissimulation I went to work!

This also demonstrates the narrator's anosognosia. He perceives his behavior as perfectly normal and healthy and not at all the behavior of a "madman."

Madmen know nothing. ... Ha! would a madman have been so wise as this? ... And have I not told you that what you mistake for madness is but over-acuteness of the sense?

His hallucinations include perceiving the old man's eye as "evil" and hearing "the groan of mortal terror" coming from the old man, as well as the "tell-tale heart" itself that the narrator believes he can heart beating while the old man sleeps. The narrator believes he hears the heart beating again after the old man is dead and buried, which ultimately drives him to reveal his crime.

The narrator claims "I loved the old man," but his obsession with murdering the old man to destroy his "evil eye" completely negates any positive feelings he has about the old man. The narrator is driven to murder by irrational hatred and rage.

His anxiety, exhibited by hyperactivity, and his paranoia are evident when he's talking with the policemen who have come to his home to investigate a report of a "shriek" heard by a neighbor during the night.

I paced the floor to and fro with heavy strides, as if excited to fury by the observations of the men—but the noise steadily increased. Oh God! what could I do? I foamed—I raved—I swore! I swung the chair upon which I had been sitting, and grated it upon the boards. ... They heard!—they suspected!—they knew!—they were making a mockery of my horror!

Despite the narrator's claims to the contrary, his behavior shows him to likely be suffering from a serious psychological disorder which, in Poe's time, would characterize him as "mad."

What disease does Edgar Allan Poe's narrator suggest that he has in the first paragraph of "The Tell-Tale Heart"?

The narrator defends himself against charges that he is insane, and he claims that his disease has, in reality, "sharpened [his] senses" and not destroyed or dulled them as one might expect disease to do. By way of evidence, he claims that he can hear "all things in the heaven and in the earth" as well as much of what happens in hell. He never says exactly what disease he believes himself to have, but he is careful to make sure that he defends himself against any charge that he is insane. However, if he believes that he is actually hearing things happening in heaven and hell and all over the earth, then what he must be experiencing are hallucinations. These hallucinations combined with his sudden violence could be the result of schizophrenia, a disease that tends to strike when a person enters their late twenties or thirties. This adult-onset disease might explain how he lived with the old man for so long and only recently grew so upset about the old man's eye.

What disease does Edgar Allan Poe's narrator suggest that he has in the first paragraph of "The Tell-Tale Heart"?

""nervous...dreadfully nervous I had been and am.  But why will you say that I am mad?" the narrator begins in "The Telltale Heart."  The narrator has most likely had a nervous (mental) breakdown and is worried about himself; he asks, "will you say that I am mad?" Added to this, he is delusional and seems to suffer from OCB, obsessive-compulsive behavior, as he is fixated on the eye of the old man, so much so that he feels compelled to do something as drastic as kill the man to be free of the "vulture eye" that makes his blood run cold.

(There is a similar question to yours; see the others in this group)

What disease does the narrator of "The Tell-Tale Heart" have?

There is never a specific disease mentioned, but my guess would be that he suffers from paranoid schizophrenia. These people are often dillusional and typically feel that others are out to get them.