Christopher Lloyd and Michael J. Fox, who played Dr. Emmett Brown and Marty McFly in the iconic "Back to the Future" film trilogy, reunited at this year's New York Comic Con nearly 40 years after the sci-fi films were released.

On Saturday, the two teamed up for a panel at the Javits Center event to talk about the saga, while fans grew teary-eyed watching the pair remember the good ol' days.

"The best part of the movie was working with Chris," Fox, 61, said of 83-year-old Lloyd, recalling when they first met. Fox, calling him a "genius," said there was "immediate chemistry" between them, referring to Lloyd as the "King of Exposition."

But reliving their sci-fi glory days onstage fell to the wayside for audiences online, who noticed Fox's evident health decline. In 1991, the Canadian-American actor, who was 29 at the time, was diagnosed with Parkinson's disease, a brain disorder that can affect motor functions and bodily movements and cause shaking and stiffness, as well as trouble with balance.

When coming onstage at Comic Con, fans spotted the tell-tale signs of the disease as Fox appeared to have difficulty walking and moved slowly — yet he still appeared cheerful.

Fans applauded Fox as an "inspiration" and said they were "crying" over the sweet embrace shared by him and Lloyd, who held up his fellow actor.

"My father-in-law battled Parkinson's for many years before passing away a few months ago. It's brutal," wrote politician Steve Marchand on Twitter. "The effort, determination & courage on display here by Michael J Fox brought me to tears. Massive respect for him."

"People in the comments keep saying it's 'so sad to see him this way.' about Michael J. Fox," tweeted another user. "I'm just so glad he's still alive, fighting, humor intact, and able to make it to another Comic Con. I'm not sad as long as he's still here."