Out singer Elton John praises Princess Diana's AIDS advocacy in a new documentary.

Diana, the first wife of Charles, Prince of Wales and the mother of Prince William and Prince Harry, died unexpectedly at the age of 36 when her car crashed in a Paris tunnel on August 31, 1997, roughly a year after her divorce from Prince Charles.

HBO will premiere Diana Our Mother: Her Life and Legacy on Monday, August 31, the 20th anniversary of her death.

The documentary follows Prince Harry and Prince William as they remember their famous mother.

In a clip released this week, John is seen with Prince Harry at the London Lighthouse Aids Center.

“[AIDS] was considered to be a gay disease,” John says. “And for someone who was within the Royal Family, and who was a woman, and who was straight – to have someone care from the other side was an incredible gift.”

As Prince Harry and Elton look at photos of Diana visiting patients, John adds: “I haven't experienced many people in my life who have that ability, but she could walk into a room of people and make them feel that everything was great.”

Diana Our Mother: Her Life and Legacy premieres Monday, August 31 on HBO.