Koirala, 75, flew to the US 10 days ago for medical treatment as his doctors advised him to have a thorough medical check-up in a well-equipped hospital after black spots were found in his lung, a couple of weeks ago.
"I am really sad to confirm the fact that cancer in Prime Minister's lungs has been confirmed by the reports of several investigations," ekantipur.Com quoted Koirala's personal physician Dr Kabirnath Yogi as saying.
Earlier, Yogi, who accompanied Koirala to the Memorial Sloan Kettering Cancer Center in New York, said the prime minister would receive radiotherapy for treatment of the localised lesion of the lung since his chronic bronchitis does not permit a major surgery of the lung.
Koirala, who underwent an operation for his tongue cancer seven years ago, also had medical follow up in the hospital.