Q. I have a problem with my fingernails. They have vertical ridges in them, rather than a smooth appearance. When I mentioned this to some of my friends, I was surprised to see how many had other problems with their fingernails, such as white streaks, cracked tips or discolored areas. Someone told me that I probably had a fungus infection. What else could cause changes in the appearance of fingernails?

A. To some degree, your fingernails are windows that can offer a view of the state of your overall health. Even though much of the folk wisdom about fingernail problems does not hold true, changes in your fingernails can be a clue to an underlying illness.

Vertical or longitudinal ridges in the fingernails are usually a simple variation of normal, a condition known as reedy nails. The cause is unknown, and they don't seem to have any worrisome significance. Occasionally, ridges can develop in people who have an underactive parathyroid gland, rheumatoid arthritis or other uncommon medical conditions.

Fungal infections of the nails are common. This type of infection develops slowly over months or even years. It can take a while to show up because of how slowly your nails grow. Fungal infections lead to brittle, discolored nails that split and lift up off the nail bed. Although they used to be difficult to treat, there are some new anti-fungal pills that are usually effective if taken for months at a time.

White streaks or white areas of the nails are also common. Although in most cases they are not related to health problems, some white bands are associated with underlying illness. Nails that are mostly white in the nail bed beneath the nail, with an outer band of pink, may occur in people who have cirrhosis of the liver. This problem is known as Terry's nails, after the doctor who discovered the cirrhosis link.

Nail beds with a white inner band and an outer band of red or brown have in rare cases been seen in people with kidney failure. Nail beds with an inner red band sometimes occur in people with heart failure. Nail beds with a blue inner band can occur in Wilson's disease--a condition in which copper builds up to toxic levels in the body.

Vertical red streaks beneath the nail are known as splinter hemorrhages. They can occur in people with bloodstream infections that affect the valves of the heart. They can also develop after manual labor or trauma.

Nails with brittle, jagged edges sometimes develop in people who have poor nutrition, iron deficiency, overactive thyroid gland, psoriasis or trauma. Nails that become concave--curving inward like a spoon--are known as spoon nails. They sometimes develop in people with iron deficiency anemia.

Tiny pits or depressions in the nails occur in people with psoriasis. In some cases, the fingernail changes are the first or only sign of this skin condition. A ladder-like series of grooves, sometimes darkened, can appear in the center of the nail from a nervous habit of flicking one finger against the nail bed of the other. This constant, low-level trauma damages the growing nail, leading to ridging, discoloration or both. However, you can also inherit fingernails with this appearance, a condition known as median nail dystrophy.

If you've had a bad illness, perhaps with a prolonged fever, you may develop what are known as Beau's lines. These are horizontal depressions across the nail that grow out as the nail grows. You may also develop what are called lines of Mees. These are white bands that develop during an illness or poisoning. Both types of lines serve as markers for when you were sick. By looking at their location, a doctor can make a good guess about how long ago the illness occurred. Illnesses that can cause these lines include pneumonia, kidney failure, heart failure, heart attack and sickle cell anemia.

Many other conditions can cause misshapen nails, including chronic infections of the nails and diseases affecting the blood vessels or nerves in the arm. The developing nail is sensitive to any changes in the health of its blood supply and the delicate nail bed from which it grows. Damage to the nail bed can have a permanent effect on the nail's appearance.

Finally, many medicines can cause discolored nails. Medications can lead to streaks of color or nails that are totally discolored. If you are taking medication and have any abnormal color of the nails, ask your doctor to check to see if any of your medicines could be to blame.

Jay Siwek, chairman of the department of family medicine at Georgetown University Medical Center, practices at the Fort Lincoln Family Medicine Center and Providence Hospital in Northeast Washington.

Consultation is a health education column and is not a substitute for medical advice from your physician. Send questions to Consultation, Health Section, The Washington Post, 1150 15th St. NW, Washington, DC 20071. Questions cannot be answered personally.