Japanese Emperor Akihito will undergo heart bypass surgery this week after a detailed examination found a heart problem had worsened, the Imperial Household Agency was quoted as saying on Sunday.
Kyodo news agency, quoting the Imperial Household Agency, said the emperor, 78, would have the surgery on February 18.
Akihito had undergone an angiogram on Saturday which showed that blood vessels had narrowed over the past year. He had been treated in hospital for three weeks in November for cold, fever and symptoms of bronchitis.
The Imperial Household Agency was unavailable for comment.
Kyodo said the test was performed after the emperor experienced difficulty while engaging in light exercise.
Television footage on Sunday showed the emperor greeting doctors after being discharged from hospital following the examination.
Akihito had surgery for prostate cancer in 2003 and suffered stress-related health issues in late 2008, including irregular pulse and stomach bleeding. The following year, the royal agency said he would cut back on official duties such as speeches and meeting foreign dignitaries.
Akihito ascended to the throne after the death of his father Hirohito in 1989. He has spent much of the past two decades working to heal the wounds of a war waged across Asia in his father's name and helped bring the monarchy closer to ordinary citizens.
Having witnessed as a boy the rise of Japanese militarism and its defeat in 1945, the soft-spoken Akihito said he wanted to deepen international understanding through visits abroad, sometimes defying protests to do so.
One of his more controversial trips came in 1992, when he became the first Japanese monarch in living memory to visit China, where bitter memories of Japan's invasion and occupation in the 1930s and 1940s run deep.
Another defining moment came in 2001 when Akihito tried to smooth relations with South Korea, at times strained by lingering resentment over Japan's 1910-1945 colonisation of the Korean Peninsula.
At a news conference marking his birthday, he said he felt a certain kinship with Korea because one of his ancestors had come from there, an unprecedented statement from a Japanese royal that made front page headlines in Seoul.
Save for such rare occasions, the Japanese imperial family is not an object of such intense public attention or media scrutiny as the British royals. It serves as important link with tradition and a comforting presence at times of distress.
Five days after last year's March 11 earthquake and tsunami that struck Japan's northeast, Akihito made a rare televised address and in April travelled to the ravaged area with Empress Michiko.
Akihito's hospital stay in November gave the Japanese a rare opportunity to see Akihito's designated successor, Crown Prince Naruhito, 51, step in for his father and perform public duties.
While Akihito's reign was defined by his reconciliation efforts, it is less clear what role the scholarly Naruhito may play, though royal commentators expect him to continue his father's efforts to reach out to ordinary citizens.
(Reporting by Chikafumi Hodo; Writing by Tomasz Janowski; Editing by Ron Popeski)