On February 1, 1908, Morton Prince — a Boston psychiatrist and pioneering figure in American psychology — published the case study that would become the most widely cited early account of multiple personality disorder (now called dissociative identity disorder). The paper, appearing in the Journal of Abnormal Psychology and later expanded into the book The Dissociation of a Personality (1909), presented the clinical course of “Miss Beauchamp,” a pseudonym used to protect the patient’s identity. That patient is now generally identified in historical accounts as Christine Beauchamp, though contemporaneous records withheld her real name in published materials. Prince’s report was notable for its systematic documentation: repeated observations of distinct personality states, the patient’s own narratives of amnesia between states, changes in handwriting and behavior, and the therapeutic interventions tried over a multi-year period. Prince framed the case within contemporary debates about hysteria, hypnotic suggestion, and the nature of the unconscious. He treated the phenomena as evidence of dissociation — a splitting of mental functions — and argued that careful clinical observation and suggestion could both reveal and influence alternate states. At the time, such phenomena had been described in older psychiatric and neurological literature and in continental European case reports, but Prince’s account was important for English-speaking psychiatry because of its detail, longitudinal follow-up, and theoretical framing. He marshaled clinical notes, transcripts, and correspondence to make a case that the alternate states were not mere role-playing or malingering but had patterns consistent with serious dissociative pathology. Historians note that Prince was both a clinician and a promoter of psychological science; his prominence helped secure wide attention for the Beauchamp case. The case influenced clinicians and researchers throughout the 20th century, shaping diagnostic thinking about what was then called multiple personality and later relabeled dissociative identity disorder (DID) in diagnostic manuals. Debate persisted, however, about etiology, prevalence, and the influence of suggestive therapeutic techniques. Some later critics argued that therapeutic suggestion and cultural factors could shape and even produce reported alternate personalities, a controversy that gained prominence in the late 20th century. Primary limitations in reconstructing the case include Prince’s use of a pseudonym, the selective nature of published clinical material, and the broader historical context in which diagnostic labels and methods differed from contemporary practice. Nonetheless, historians and clinicians generally agree that Prince’s Beauchamp report represents an early, influential, and well-documented clinical account that helped define the phenomenon for subsequent generations. In summary, February 1, 1908, marks the publication date of Morton Prince’s influential case study of “Miss Beauchamp,” commonly regarded as the first confirmed and systematically documented case of multiple personality disorder in modern psychiatric literature. The case shaped clinical and theoretical discussions about dissociation for decades and remains a central historical reference in studies of dissociative identity phenomena.