Untersuchungen über eine Nematode (Spiroptera sp. n.) und deren Fähigkeit papillomatöse und carcinomatose Geschwulstbildungen im Magen der Ratte hervorzurufen.

Fibiger demonstrated that the roundworm, which he called Spiroptera carcinoma (but is correctly named Gongylonema neoplasticum), could cause stomach cancer (squamous cell carcinoma) in rats and mice. His experimental results were later proven to be a case of "mistaken conclusion."

In 1926 Fibiger was awarded the 1926 Nobel Prize in Physiology or Medicine "for his discovery of the Spiroptera carcinoma."