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Ludwig Wilhelm Carl Rehn (April 13, 1849, Bad Sooden-Allendorf – May 29, 1930) was a German surgeon. Rehn was born in 1849, in the village of Allendorf, the youngest of five children. After the visiting the convent school in Bad Hersfeld, he studied medicine at the University of Marburg from 1869 to 1874, where he became a member of the student corps Hasso-Nassovia.
Rehn served as a volunteer in the Franco-Prussian War of 1870-71. In 1875 he received his doctorate from University of Marburg and started to practice at Griesheim near Frankfurt am Main and later in Rödelheim. While there in 1880, he carried out the first thyroidectomy. From 1886 on he was a surgeon and late head of department and manager of the surgical facility of the urban hospital in Frankfurt am Main.
In 1895 Rehn reported cases of bladder cancer with workers, and people downriver of, the local aniline factories. The risk from aromatic amines, particularly benzidine and a-and ß-naphthylamine, was not well-established until the 1950s.
In 1914, he was appointed professor of surgery at the newly founded University of Frankfurt am Main. During the World War I he served as a surgeon general. Rehn was also a member of the scientific senate of the Kaiser Wilhelm Academy in Berlin.
Rehn was the first to successfully conduct heart surgery, when on September 9, 1896 repaired a stab wound suffered by 22-year-old gardner Wilhelm Justus. 
He died in 1930. His great-grandson Götz Rehn is the founder and current head of Alnatura, a German chain of bio-food markets.
The Ludwig Rehn-Prize, donated in 1973 by the Chamber of Industry and Commerce of Frankfurt am Main, has been awarded since 1974 for excellent scientific work in the area of the general surgery. Additionally, Ludwig Rehn-Strasse in Frankfurt am Main was named after him.
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- Blatchford JW. Ludwig Rehn: the first successful cardiorraphy. Ann Thorac Surg 1985;39:492–5.