Russian surgeons thought Artyom Sidorkin had a cancerous tumour when he started coughing up blood and complaining of agonising chest pains. When they opened the man up, they found a perfectly formed fir tree growing in his lung.
Artyom Sidorkin, 28, from the Urals region of Russia, is believed to have inhaled a seed which then sprouted inside him.
Doctors were convinced he had cancer after he came to them complaining of agonising chest pains and coughing up blood.
An X-ray showed what was believed to be a tumour, and he was rushed to the operating theatre.
"We were 100% sure," said surgeon Vladimir Kamashev from Izhevsk in the Urals.
"We did X-rays and found what looked exactly like a tumour. I had seen hundreds before, so we decided on surgery."
Before removing the major part of the man's lung, the surgeon investigated the tissue taken in a biopsy.
"I thought I was hallucinating," said Dr Kamashev.
"I asked my assistant to have a look: 'Come and see this - we've got a fir tree here'."
"He nodded in shock. I blinked three times as I was sure I was seeing things."
The 2in (5cm) spruce, which was said to be touching the man's capillaries and causing severe pain, was removed.
Mr Sidorkin, now recovering after the op, said: "To be honest I did not feel any foreign object inside me. But I'm just so relieved it's not cancer."