Fischer exhibited eccentric behavior throughout his life, including rigid routines, unusual dietary habits, and a strong aversion to germs. These behaviors were often attributed to his intense focus on chess, but also raised questions about his mental state.

Fischer's frequent demands regarding tournament conditions and his occasional withdrawals from competition were sometimes seen as signs of instability or psychological issues.

In his later years, Fischer became increasingly paranoid and espoused conspiracy theories. He claimed to be a victim of persecution and made accusations against various individuals and institutions.

Fischer's anti-Semitic and anti-American statements were widely condemned and raised concerns about his mental state. Some suggested that he may have been suffering from a mental illness.

While no formal mental health diagnoses were ever publicly confirmed, many have speculated about possible conditions such as paranoid schizophrenia or Asperger's syndrome based on observed behaviors and statements. These speculations remain unconfirmed.