At least half a million New Yorkers have diabetes, and many of them are at risk for blindness, kidney failure, amputations, and heart problems because they are doing a poor job controlling their illness.

The question is, how much privacy are they willing to give up for a chance at better health?

A century after New York became the first American city to track people with infectious diseases as a way to halt epidemics, officials here propose a similar system to monitor people with diabetes, a noncontagious foe.

Conceived after a sharp rise in diabetes deaths over the past 20 years, the plan would require medical labs to report to the city the results of a certain type of test that indicates how well individual patients are controlling their diabetes.

“There will be some people who will say, ‘What business of the government is it to know that my diabetes is not in control?'” the city’s health commissioner, Dr. Thomas Frieden, said.

The answer, he said, is that diabetes costs an estimated $5 billion a year to treat in New York and was the fourth leading cause of death in the city in 2003, killing 1,891.

By pinpointing problem patients, then intervening ever so slightly in their care, Dr. Frieden said the city can improve thousands of lives. “I don’t think we can afford not to do anything,” he said. The Board of Health vote on the proposal isn’t likely until at least September, but it has already attracted attention from other public health experts and privacy advocates.

The list of illnesses reported to public health authorities has grown over the years, but it still is almost entirely contagious diseases, like HIV, or conditions related to environmental toxins, like lead poisoning.

Diabetes is different, threatening no one but the people who have it.

“This isn’t smallpox,” an attorney who represents health-care groups concerned with medical privacy, James Pyles, said. “The state, or the city in this case, does not have a compelling interest in the health of an individual that overrides that individual’s right to privacy.”

Mr. Pyles praised the intent of the program, but said unless diabetics are asked for their consent, it would be “an outright violation of the constitutional right to privacy” for the government to obtain their identities.

The city’s program wouldn’t initially get consent to collect data, but would allow patients to opt out later. The database would also be tightly controlled, off-limits to anyone but department staff, the patients, and their doctors, health officials say.

Over time, doctors could receive letters, telling them whether their patients have been getting adequate care. People who skip checkups might get a note from their doctors reminding them of the dangers of untreated diabetes.

The plan is akin to the surveillance system put in place in 1897 to fight tuberculosis. At first, doctors were outraged they had to report TB cases to the government, but it became a model after deaths plummeted.