When she refused a sex verification test at age 24, she was banned from competition by the International Association of Athletics Federations in 1950.

[6] On 13 July 1950, Dillema was stopped on her way to an international meeting in France by the Dutch athletics authorities and expelled for life from competition.

After her death, a forensic test on body cells obtained from her clothing showed signs of a Y-chromosome in her DNA.

She may have been a genetic mosaic, having cells with either 46,XX (female) or 46,XY (male) chromosomes, in approximately a one-to-one ratio, in her skin.

The SRY-gene on the Y is the testis determining factor, so Dillema may have had infertile testes or ovotestes palpable in her groin.

Higher testosterone levels were considered unfair towards the competition, despite being an entirely natural feature of Dillema's physiology.

Shortly after Dillema's death, the director of the Royal Dutch Athletics Federation apologized to her family for the manner of her suspension and her personal best in the 200 metres was reinstated.