Thirteen years ago, Leroy Grayson noticed a small white spot on his ankle. At first, he ignored the spot, which he thought might have been caused by a bruise. Then white spots began to appear around his nails and on the back of his hands. He also ignored these, suspecting a household detergent was causing the problem.

But as time went on, he simply couldn't ignore what was happening: His normally black skin was turning into a patchwork of large, unsightly permanent white blotches. The blotches covered all parts of his body, especially his face, chest, back and neck.

"I didn't know what was happening to me," recalls Grayson, who lives in McLean. He felt fine physically, and he continued to think that the white spots were somehow self-inflicted.

Finally, Grayson went to see a physician, who told him that he had vitiligo (vih-tih-LIE-go), an ancient disease that affects about 2 percent of the world's population. It strikes individuals of all races, but it is one of a number of skin diseases that are more pronounced, and in some cases more common, in blacks.

Like all these diseases, vitiligo is not life-threatening. Most patients with the disease are otherwise in good health and remain that way, says Dr. Rebat Halder, director of the Vitiligo Center at Howard University.

But vitiligo is "psychologically and socially devastating," says Halder. Victims of vitiligo are often forced to alter their lives because of the disease. For instance, one of Halder's patients was employed as a cashier in a grocery store, but, because she had vitiligo on her hands, customers refused to give her money because they thought she was unclean.

Halder says vitiligo -- whose name is derived from the Latin word vitium, meaning blemish -- is thought of as a sign of uncleanliness or unholiness in parts of the world. For example, vitiligo is a deterrent to marriage in India, where it is often confused with the stages of leprosy associated with white skin patches.

Vitiligo strikes suddenly at any age and its onset cannot be prevented. About one third of patients have some relatives with the disease, but it is not necessarily transmitted directly to the next generation. About 6 percent of vitiligo patients also have other abnormalities such as thyroid disease, pernicious anemia, rheumatoid arthritis or diabetes.

Researchers are still unsure what triggers the condition, but they believe vitiligo is an autoimmune disease. This means that the body wages a battle against itself, enlisting the help of antibodies -- substances designed to kill foreign organisms in the body, such as viruses -- to destroy melanocytes. No one knows why only certain parts of the skin lose pigment and not others.

Melanocytes are the cellular factories in the uppermost skin layers that produce the brown-colored pigment called melanin. In areas where melanocytes are destroyed, the skin appears pale or chalk white. About 80 percent of vitiligo patients have antibodies in their blood that are targeted specifically against melanocytes.

To treat vitiligo, doctors use ultraviolet radiation along with an orally administered drug called psoralen to stimulate pigment production.

The drug increases the efficiency of ultraviolet light in stimulating the scattered melanocytes that still exist in deeper layers of the skin. Once stimulated, the melanocytes migrate to the upper layers of the skin and begin laying down new pigment.

This treatment succeeds in restoring pigment to the skin in about half the vitiligo patients who receive it. But it takes a long time to work, Halder says -- 12 to 18 months of treatment or longer.

After 2 1/2 years of treatment, more than half the white spots on Grayson's body have been restored to their normal color. However, Grayson, 61 and retired from his job with the Arlington County sanitation department, is resigned to the fact that his lips will never return to their natural hue.

Melanocytes are present in the same number and in the same location in all races. In black skin, however, the melanocytes pack the pigment more densely in melanin-containing sacks called melanosomes.

Those densely packed sacks in black skin act as a natural sunscreen, filtering out the harmful ultraviolet radiation that can cause skin cancers. Consequently, blacks rarely develop sun-induced skin cancers and are at least four times less likely than whites to develop malignant melanoma, a life-threatening disease increasing in many parts of the world.

The special skin characteristics that protect blacks from some forms of cancer can make other skin diseases more noticeable. These skin diseases are seen in whites as well as blacks, but appear different because of skin color.

For blacks, the skin may appear darker after an injury when irritation stimulates melanocytes in upper skin layers to form dark spots on the skin.

Blacks are less prone than whites to severe acne, but common acne tends to leave dark brown spots that may be permanent in black skin, says Dr. Gary Brauner, a skin disease specialist at Albert Einstein College of Medicine in New York. Occasionally these spots may fade spontaneously or return to a lighter color in response to bleaching medicine. Some pigment granules, however, may drop into the skin's deeper layers, forming a tattoo.

Another condition -- particularly common among black children -- is pityriasis alba, a kind of eczema leaving white flakes on the skin. This condition is caused by dry skin and is easily relieved by moisturizers.

One disease that is seen frequently in blacks may have little to do with the composition of the skin itself. For some unknown reason, keloids occur 10 to 20 times more commonly in blacks than in whites, says Dr. Philip Bailin, chief of dermatology at the Cleveland Clinic Foundation. Keloids are scars that grow far beyond the original borders of a wound, sometimes forming grotesque masses of tissue weighing several pounds.

"We have had several patients with keloids extending from one ear around the chin to the other, making it almost impossible for them to eat," says Bailin. Other patients have had keloids that hang off the back or arms like mushrooms.

Any type of injury can cause keloid development. Keloids commonly occur on the ear lobes (due to ear piercing), the central chest, upper back and shoulders, and jaw line. Both men and women are affected, and the propensity to develop keloids may be inherited. Treatment is usually difficult, since about half of them tend to recur after they are removed, says Bailin.

Another skin disease that has nothing to do with pigment but occurs almost exclusively in blacks is called pseudofolliculitis barbae, commonly known as razor bumps. As many as 60 percent of black men who shave have this condition, says Dr. A. Melvin Alexander, a Washington dermatologist.

In blacks, all body hair is naturally curved or curled. When beard hair is cut with a razor, the sharpened hairs can hook back into the skin and become ingrown, causing painful ulcerations. The best cure for the disease is simply to let the beard grow for a month, which allows enough time for the bumps to heal.