Heavy rain flooded a gorge filled with hikers in the southern region of Calabria, killing at least eight, Italy's civil protection agency said.
At least another five were missing.
The prefect's office said 23 people were rescued from the flash flood in the Raganello Gorge.
They included a 10-year-old boy treated for hypothermia who was among seven that the civil protection agency said were hospitalised.
It was unclear how many people were missing but there were at least 36 hikers in two organised groups inside the 12km long gorge, a popular aquatic trekking spot.
TV images show rescuers scaling down the side of a steep rock face to bring hikers to safety.
Guides are not required, making it impossible to know how many people were on their own inside the canyon.
"The problem is we don't know how many people were knocked over by this flood," Carlo Tansi, the head of civil protection in Calabria, told Sky TG24.
"This is a split in the terrain that is very tight and high."
At its most narrow point, it is just metres wide and with walls varying from 400m to 700m in height.
Luca Franzese, of the alpine rescue squad in Calabria, said the height of the flood waters was some 2 ½m deep.
"The wave of flooding of the Raganello stream happens often in the winter, but it has never happened in the summer, when the stream is very popular among tourists," Franzese told the news agency ANSA.
The gorge on the eastern side of the Pollino National Park boasts aquatic trekking along the stream that cuts through the massive rock, where hikers pass by water falls, water tubs and natural slipways, according to a website.
The gorge is broken down into three sections, the upper, mid and lower canyons, with hikes averaging between 2 ½ and 3 ½ hours and varying in difficult.
Guides include spelunkers and rescue instructors, who supply helmets, wet suits and other equipment.