De Graaf was born in Schoonhoven. He studied medicine in Utrecht and Leiden. There his co-students were Jan Swammerdam, Niels Stensen and Frederik Ruysch, one of their professors was Franciscus Sylvius. (All of them were interested in the organs of procreation). He submitted his doctoral thesis on the pancreas, and went to France where he obtained his medical degree from the University of Angers. While in Paris, he also turned to the study of the male genitalia, which led to a publication in 1668. Back in the Netherlands in 1667, De Graaf established himself in Delft. Since he was a Catholic in a mainly Protestant country, he was unable to follow a university career. After the early death of a son, De Graaf died in 1673 at age 32 and was buried in the Oude Kerk in Delft. The reason for his death is unknown, he was, however, affected by his controversy with Swammerdam (v.i.) and the death of his son. Recent speculation that he may have committed suicide is entirely unfounded. A few months before his death De Graaf recommended, as a member of the Royal Society in London, that attention be paid to Antonie van Leeuwenhoek and his work on the improvement of the microscope.