The clinic had been damaged during a fire and the Prince was aware that a royal visit would lift spirits.
His mother Diana supported the Royal Marsden as president from 1989 until her death – a role that her elder son willingly took on in 2007.
On Thursday, the 29-year-old will take his bride Kate to open a new children's cancer centre in the hospital that was so close to her heart.
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His encouragement for the staff and their fundraising efforts has never flagged.
A few months after that fire in 2008, William returned to catch up on the repairs and meet surgeons and nurses who calmly carried on operating after the blaze broke out.
And the hospital was one of the charities to benefit from Concert for Diana, a celebratory event staged by her boys ten years after her death.
Chief executive Cally Palmer was invited to be a guest in Westminster Abbey when the future King made Kate his wife.
She said: "We are delighted that Their Royal Highnesses The Duke and the Duchess of Cambridge have chosen to officially open the new Oak Centre for Children and Young People at The Royal Marsden as one of their very few official engagements since their wedding."