This article presents the history of the discovery of compression of the median nerve in the carpal tunnel without an identifiable cause as a distinct clinical entity. By analyzing primary sources, we show that, at the beginning of the twentieth century, physicians described patients with paresthesias and numbness in the hands, most prominent at night, accompanied by bilateral symmetrical atrophy along the radial side of thenar eminence. At the time, the 2 most influential hypotheses regarding etiology were, first, compression of the lower trunk of the brachial plexus by a cervical or first rib, and second, compression of the thenar branch of the median nerve as it passes beneath the anterior annular ligament of the wrist. The condition was named syndrome of partial thenar atrophy and was considered a distinct clinical entity. In 1946, after extensive analysis, neurologist Walter Russell Brain concluded that both sensory and motor symptoms of the syndrome were caused by "compression neuritis" of the median nerve in the carpal tunnel. At his suggestion, surgeon Arthur Dickson Wright performed decompression of the nerve by "an incision of the carpal ligament," with excellent results. Brain presented this work at the Royal Society of Medicine in London in 1946 and published his landmark paper in Lancet the following year. In so doing, he established the basis for the disease we know today as idiopathic carpal tunnel syndrome. Unfortunately, in 1947, Brain did not realize that another "condition" with the same clinical picture but without atrophy of the thenar muscles, known as acroparesthesia at the time, was actually the same disease as syndrome of partial thenar atrophy, but of lesser severity. As a result of Brain's influence, 7 other papers were published by 1950. Between 1946 and 1950, there were at least 10 papers that presented, in total, 31 patients (26 women) who exhibited symptoms of compression of the median nerve without an identifiable cause and underwent section of the transverse carpal ligament.