People who survived the atomic bombs dropped by the US on Hiroshima and Nagasaki in 1945 are still suffering health problems, a study reports.
The mushroom cloud seen over Nagasaki on 9 August 1945
The younger they were at the time, and the more radiation they were exposed to, the higher their risk of illness.
The Radiation Effects Research Foundation looked at thyroid conditions, known to be linked to radiation exposure.
The study is published in the Journal of the American Medical Association.
THE BOMBINGS OF HIROSHIMA AND NAGASAKI
6 August 1945 - US drops atomic bomb on Hiroshima
9 August - Nagasaki is bombed
Around 214,000 people were killed in the bombings
Japan surrendered to the Allies on 14 August 1945
People who have been exposed to radiation are known to be at an increased risk of malignant and benign thyroid nodules, but few studies have followed them for long periods.
Studies of children, including those exposed to radiation after the Chernobyl disaster, have shown the younger a person is the higher the risk of thyroid conditions.
Between 2000 and 2003, the Japanese researchers looked at the incidence of thyroid diseases among 3,185 people, with an average age of 70, who had been in the cities when the bombs hit.
The team also looked at people's level of exposure to radiation - which was possible because all those studied had been followed throughout their lives.
Young 'at greater risk'
Just under 45% - 1,833 - of those studied had malignant tumours, nodules [lumps on the thyroid] and cysts.
Those who were aged under 20 when the atomic bombs dropped had a higher risk of disease than those who had been older.
The researchers, led by Dr Misa Imaizumi, wrote in JAMA: "The present study revealed that, 55 to 58 years after radiation exposure, a significant relationship existed in the prevalence of not only malignant thyroid tumours but also benign thyroid nodules and that the relationship was significantly higher in those exposed at younger ages.
"Thus, the effect of radiation on the thyroid nodules may exist long after radiation exposure in atomic bomb survivors."
Sarah Darby, professor of medical statistics at Cancer Research UK, said: "This is a unique survey that provides an important insight into the relationship between ionizing radiation and the risk of thyroid cysts and nodules, including cancer.
"These conditions are rarely fatal, and some people with a thyroid cyst or nodule do not experience any symptoms.
"Therefore, it is difficult to collect information on the relationship between radiation exposure and the subsequent risk of thyroid disease that is free from any bias."