Wilder Penfield (1891–1976) is widely regarded as a dominant figure in 20th century neurobiology for his singular contributions to the structure-function relationship of the brain, his discovery of the language function of the supplementary motor area, the discovery (with Herbert Jasper and Brenda Milner) of the anatomy of recall, and his pioneering work in the surgical treatment of focal epilepsy. But another of his significant discoveries has escaped notice: the recognition that focal microgyria can generate epileptic seizures, and that these can be treated surgically. Penfield discussed the case of the patient through which this discovery was made during his Shattuck Lecture to the Massachusetts Medical Society in 1939. As Penfield gave only a fragmentary account of this case, the patient's chart was retrieved from the Montreal Neurological Institute archives, and his operative note and brain map, intraoperative photographs, and the histopathological and cytological examinations of the resected specimen were reviewed. Based on these primary sources, this paper provides a complete, detailed account of the first case in which microgyria was recognized as a cause of focal epilepsy, which was successfully treated surgically.