Worldwide, there were 1.1 million cases of prostate cancer and 307,000 deaths from it in 2012, the latest year data were collected by the International Agency for Research on Cancer. In the United States, about 181,000 cases and 26,000 deaths are expected in 2016. The average age at diagnosis is 66 in the United States, and the disease rarely occurs in men under 40. Most men who have prostate cancer do not die from it, according to the American Cancer Society.

The disease often grows very slowly — but not always. Some cases are potentially deadly, but tests cannot always tell which ones. The uncertainty leaves many men in a quandary, particularly because of the bowel, bladder and sexual problems from treatment.

In 2012, the United States Preventive Services Task Force, an independent panel of experts picked by the government, recommended against routine screening for prostate cancer with the PSA test. The group said screening finds many tumors that may never have harmed the patient, and leads too many men into unneeded surgery or radiation, with their troubling side effects.

Dr. Hamdy’s team set out to address the quandary. They studied 1,643 patients in Britain ages 50 to 69 who had early prostate cancers, found with routine PSA testing and then a biopsy if the PSA was abnormal. All the cancers were localized, meaning they were confined to the prostate and had not spread to nearby tissue outside the gland, or to distant organs.

The patients had PSA measurements of 3 or higher, and about three quarters had a Gleason score of 6; the rest had higher Gleason scores. Gleason scores are a measure of aggressiveness and range from 6 to 10 in cancers, with higher scores being worse.

The patients were then assigned at random to one of three groups: A third had surgery, a third had radiation, and a third had active monitoring.

Though death rates from the cancer did not differ, more men on active monitoring had progression. The disease spread to distant parts of the body in 33 men on monitoring, 13 who had surgery and 16 who had radiation. The differences were statistically significant.