Armstrong, trying for a record sixth straight Tour title, was thrown from his bike but not hurt in a crash involving a number of cyclists about 20 minutes into Friday's sixth stage, a 122-mile run from Bonneval to Angers.
But the fall didn't derail Armstrong's bid for another Tour title.
He quickly got back in the race and, with help from his U.S. Postal Service teammates, caught up with the pack. He was not seriously hurt, but appeared to have a scrape on his right leg.
The spill was the first of this Tour for the 32-year-old Texan and came a day after he said he was worried about crashing.
"In this race, I'm always scared, always nervous," he said. "The last two or three days for me, personally, have been really, really nerve-racking.
"It's a stressful race."
Boonen won a sprint finish Friday, speeding past Cofidis' Stuart O'Grady of Australia and T-Mobile rider Erik Zabel of Germany. Thomas Voeckler of France retained the overall leader's yellow jersey.
American Tyler Hamilton, a former teammate of Armstrong who now rides for Phonak, blew a tire, but caught up with the main group near the 36-mile mark.