It has been suggested that beneath the sunny personality and enormous productivity of Sir William Osler (1849–1919) lurked a deep sorrow. A longstanding rumor suggests this sorrow was a prolonged affair with an older first cousin, Marian Osler (Bath) Francis (1841–1915), which resulted in Osler’s paternity of William Willoughby Francis (1878–1959). Osler treated W.W. Francis like a son. Francis after Osler’s death devoted the rest of his life to preserving Osler’s memory. Osler’s great-great niece Anne Wilkinson (1910–1961) strongly endorses this rumor; Osler scholar George T. Harrell (1908–1999) strongly suggests its truth, while Osler biographer Michael Bliss (1941–2017) dismisses it. Evidence supporting the rumor includes ample opportunities for mating, biographical details, and phenotypic resemblances that are hard to explain otherwise. Barring a DNA-based family study, the truth will probably never be known.