Niki Lauda is back in intensive care.
The F1 legend and Mercedes team chairman had been holidaying in Ibiza following his long recovery from a life-saving lung transplant operation.
But according to Kronen Zeitung newspaper, 69-year-old Lauda had to fly back to Vienna after contracting a serious flu illness.
The newspaper cited "sources" who said Lauda caught the flu because his immune system was weakened following his medical ordeal.
"There was an influenza case in his family, and he is treated in an intensive care unit with us," a spokeswoman for the AKH hospital confirmed.
Osterreich newspaper said Lauda should be able to leave hospital "in three to four days".
"He had to go back to Vienna because there were no specialists in Ibiza familiar with lung transplants," the report added.
"The situation was serious in the first days of the new year, but Lauda is already doing much better again."