Surgeons left a two-year-old boy infertile after a blunder during an operation to treat an undescended testicle.
The boy, 2,was admitted to Bristol Royal Hospital for Children earlier in the week to treat the condition known as cryptorchidism.
But surgeons operated on the wrong testicle, with his father telling the BBC that they had "castrated him".
The University Hospitals Bristol NHS Foundation Trust apologised and said it has launched an investigation.
The boy's father, whose name has not been revealed to protect his son's identity, said his son's undescended testicle was discovered during a routine check-up.
The toddler was referred to a specialist and on Monday he was booked in for an operation at the Bristol Royal Hospital for Children, which would last around 30 minutes.
The family said they were told it was "just a minimal operation" with "minimal risk" and it would be over in around 30 minutes
"We were waiting and waiting," his father told the BBC.
"After two and a half hours the manager, surgeons and consultants they came and I knew something was not right.
"Me and my wife started panicking, they called us into the office and told us things didn't go right and the operation wasn't a success."
The couple said they were told a surgeon had mistakenly inserted a camera "into the wrong side" and now their son's healthy testicle would "never work".
"I was very distressed, it was an awful disaster for a simple operation. They destroyed everything and they ruined my son," his father said.
"They castrated him and now my son's future life has dramatically changed."
The boy's mother said it was "absolutely horrible" what the surgeons had done.
"They broke my heart and they basically destroyed his future," she said.
"I can't find the words to explain how I'm feeling - there are no words. Even tears, I have no more tears.
"We just hope for a miracle, this is what we hope."
In a statement, Trust medical director Dr William Oldfield said: "On behalf of the Trust, I am deeply sorry that a serious incident has occurred in the treatment of a young child in our care and would like to offer our sincerest apologies.
"As soon as our staff realised what had happened they met with the family to offer their apologies and explain what had happened.
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"We take patient safety and standards of clinical care very seriously and have begun a thorough investigation into this matter and will work with the family throughout this process.
"I would again like to offer my apologies to the family for this incident."
