The nature and cause of President Abraham Lincoln's unusual physical features have long been debated, with the greatest attention directed at two monogenic disorders of the transforming growth factor β system: Marfan syndrome and multiple endocrine neoplasia type 2B. The present report examines newly discovered phenotypic information about Lincoln's biological mother, Nancy Hanks Lincoln, and concludes that (a) Lincoln's mother was skeletally marfanoid, (b) the President and his mother were highly concordant for the presence of numerous facial features found in various transforming growth factor β disorders, and (c) Lincoln's mother, like her son, had hypotonic skeletal muscles, resulting in myopathic facies and 'pseudodepression'. These conclusions establish that mother and son had the same monogenic autosomal dominant marfanoid disorder. A description of Nancy Hanks Lincoln as coarse-featured, and a little-known statement that a wasting disease contributed to her death at age 34, lends support to the multiple endocrine neoplasia type 2B hypothesis.