At least 500,000 New Yorkers have diabetes, many of them at risk of blindness, kidney failure, amputations and heart problems because they are doing a poor job of controlling their illness.

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

A hundred years after New York became the first U.S. city to track people with infectious diseases as a way to stop epidemics, New York officials have proposed a similar system to track people with diabetes, a noncontagious disease. Introduced 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 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?'" said Dr. Thomas R. Frieden, the city's health commissioner. 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.

The list of illnesses reported to public-health authorities has grown over the years, but it is still almost entirely contagious diseases, such as HIV, or conditions related to environmental toxins, such as lead poisoning. Diabetes is different, threatening no one but the people who have it.

"This isn't smallpox," said James Pyles, a lawyer who represents health-care groups concerned with medical privacy. "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." Pyles said that 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 program would not at first get consent to collect data but would allow patients to opt out later. The database would also be 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 American Diabetes Association supports New York's plan. It could be a great tool for doctors who find it difficult to track patients over long periods, said Dr. Nathaniel Clark of the ADA.