Diabetes has cost Robert Heard dearly. His left leg was amputated below the knee two years ago. Kidney failure -- with its special diets, thrice-weekly dialysis and ever-present threats of infection -- is worse.

''It breaks you down mentally,'' says Heard, 32, who has been waiting for a second kidney transplant since the first, donated by his uncle, failed four years ago.

Heard was diagnosed with Type 1 diabetes at age 10, and said he managed it well for 15 years until, as an adult, he changed jobs and lost his health insurance. Unable to get the medicines he needed, he lost control of his blood sugar levels. Like many people with an uncontrolled chronic illness, Heard regularly ended up in the emergency room. And like more and more diabetics, his kidneys finally gave up.

Ask someone about diabetes, and eye problems or heart disease often come to mind. That's a correct association, but so is kidney failure.

The number of Americans diagnosed with kidney failure increased five times over the past three decades, a period that also saw a staggering rise in the number of Americans with diabetes.

Nearly half of all kidney failures in this country are now caused primarily by diabetes, according to a leading federal health agency.

''The link is incredibly powerful,'' says Richard C. Wender, professor of medicine at Jefferson Medical College in Philadelphia. ``Diabetes is the leading cause of kidney failure in the U.S.''

The United States spends $34 billion a year on kidney failure. And the bulk, more than $20 billion a year, is covered by taxpayers through Medicare.

About 24 million people nationwide have diabetes, federal authorities say, and 57 million more are prediabetic -- with elevated blood sugar levels that put them at high risk for the illness. That means 81 million people, about one in four Americans, are either diabetics or potential ones.

For doctors, patients and policymakers, the health-care system's inability to stop preventable kidney damage from diabetes is frustrating. With good primary care, diabetes can usually be managed, or even spotted and stopped before it fully develops.

But the system doesn't pay well for preventive care. Many patients aren't educated on how to manage their conditions. Drugs can be expensive, and many people lack good access to care.

Even those who see their doctors regularly could get in trouble because good diabetes care takes time, a commodity that few caregivers have to spare.

Kidney failure can be caused by either major form of diabetes: Type 1, once known as juvenile onset, arises from the body's inability to make insulin, a key hormone that regulates the body's blood sugar levels. Type 2, the more common, can develop at any age, and is caused by the body's failure to use insulin. Both can damage the kidneys and cause death if left untreated.

Each kidney is essentially a giant filter made up of roughly 1 million smaller filters known as glomeruli. These tufts of tiny vessels remove waste, but not nutrients, from blood.

Over many years, the excess sugar of diabetes harms the membranes of those vessels. An early sign of kidney disease is the appearance of tiny amounts of the protein albumin in the urine.

''If the filter is not working, you can lose a lot of the good stuff into your urine,'' says Mitchell A. Lazar, director of the University of Pennsylvania's Institute for Diabetes, Obesity and Metabolism.

How can you avoid all this?

''The best strategy is to prevent diabetes -- and the most effective way in the majority of patients,'' said Jefferson's Wender, ``is to maintain a normal weight and exercise regularly.''

A better diet may help, including lots of fruits and vegetables.

So can controlling high blood pressure, a common precursor to kidney problems.

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