A man, woman and child have died and a seven-year-old boy is fighting for his life after getting into difficulty in the water.
Police Scotland said they were called to a report of concerns for people in the water near Pulpit Rock, Loch Lomond, south of Ardlui, on Saturday evening.
A 41-year-old man, a 29-year-old woman and a nine-year-old boy were pronounced dead at the scene.
The seven-year-old boy was rushed by ambulance to the Royal Hospital for Children in Glasgow, where he is currently in intensive care.
A Police Scotland statement said it received the call at about 6.40pm on Saturday.
It added: "A multi-agency operation took place and sadly three people were pronounced dead at the scene.
"Formal identification has still to take place but the family of a 41-year-old man, 29-year-old woman and nine-year-old boy have been made aware."
A report into the incident will be sent to the procurator fiscal.
It brings the number of people who have died in Scotland after getting into difficulty in the water to five in just 24 hours.
An 11-year-old boy was pronounced dead at the scene after being found in the river at Alexander Hamilton Memorial Park in Stonehouse, South Lanarkshire, on Saturday afternoon.
A 16-year-old boy died in the water at Balloch Country Park, at the south end of Loch Lomond, on Friday.
Assistant Chief Constable Mark Williams said: "The number of deaths in open water this weekend is hard to comprehend and my thoughts are with the families and loved ones of those affected.
"The warm weather can make open water swimming and paddling very inviting but it is extremely dangerous, even for the most experienced swimmers or supervised children. The conditions can change very quickly and there are often hidden risks like deeper water and strong currents.
"The message I want to send to everyone is exercise extreme caution. It is better to keep a safe distance from water if possible. Tragically, this weekend has highlighted that open water is very, very dangerous. If you see someone in the water and distressed call 999 immediately. Remember, you could get into difficulty yourself trying to help so please take care and seek help as soon as possible."
