Q. In recent months I've noticed a number of dark moles on my neck and upper chest. They range from dot size to one quarter inch in diameter. They're not bothersome, but I've wondered whether I should worry about them being a sign of skin cancer. I'm 28 years old and have a very light skin tone. Incidentally, I spend very little time out in the sun.

A. Moles such as yours are very common. Normally, young adults have up to 25 moles, which doctors call nevi. Most moles are benign, but occasionally one may turn into a potentially lethal type of skin cancer known as a malignant melanoma.

Benign moles usually have a smooth, regular shape and uniform color. They may or may not have hairs growing out of them, and generally don't cause any symptoms, such as pain, itching or bleeding.

Malignant melanomas, on the other hand, tend to have irregular, sometimes notched borders, and uneven color. They're often dark brown or black, but mixed in are shades of blue, red or white. Warning signs that should prompt a visit to your doctor are a change in the shape or color of a mole or symptoms like bleeding.

A special type of mole that bears careful watching is the giant hairy nevus or bathing trunk nevus, so called because it's large and often located in the area covered by bathing trunks. These moles, present from birth or infancy, are usually larger than eight inches and have a dark, hairy, somewhat rough surface. Parents of children with giant nevi, and later the children themselves, should be instructed in watching these moles for unusual changes.

Other types of moles that have a higher risk of turning malignant are dysplastic nevi. These resemble malignant melanomas in having irregular shapes and uneven color. They tend to run in families and occur in large number, up to 100 or more. These also need to be closely watched.

In general, I recommend that people have a thorough skin exam as part of a routine medical check-up, especially when they're older and have had many years of sun exposure. In the meantime, I recommend using sunscreens to reduce the risk of skin cancer.

Q. What exactly is chlamydia? I've heard that it's similar to gonorrhea, that the symptoms are the same, and that the only way to distinguish between the two is to be tested for both. Is this true?

A. Chlamydia is a germ that can cause an infection resembling gonorrhea, but treatment for the two is sometimes different. Because it is one of the most common infections passed by sexual contact, it's important to get the right diagnosis and treatment for this condition.

Besides causing sexually transmitted disease, chlamydia is known for causing an eye infection called trachoma. Although this infection is virtually unheard of in the United States, it's the leading cause of blindness in the world. Chlamydia is associated with other conditions, including infections of the groin lymph nodes, liver and rectum. But here I will limit the discussion to the similarities between chlamydia and gonorrhea.

As a sexual infection, chlamydia is becoming a nationwide epidemic. In men, chlamydia most often causes urethritis, an infection of the urine channel. Symptoms are similar to gonorrhea, with discharge of pus from the penis and burning with urination.

In women, chlamydia can cause infection of the cervix, uterus and fallopian tubes. Cervical infection produces a vaginal discharge and inflammation of the cervix, which may make you have spotting after intercourse. Infection of the fallopian tubes, known as pelvic inflammatory disease, gives you lower abdominal pain and fever. Once you've had a fallopian tube infection, you're more likely to have an ectopic pregnancy (pregnancy in the tube) or be infertile. All of these problems are similar to those caused by gonorrhea.

Chlamydia infection during pregnancy poses special problems to both mother and baby. After delivery, women are at risk of infection of the uterus, causing pain, fever and other complications. Newborn infants can get the infection during vaginal birth and later come down with an eye infection (conjunctivitis) or pneumonia. This is another way in which chlamydia resembles gonorrhea, which also can cause an eye infection in newborns.

Until recently, chlamydia was difficult to diagnose, because culture tests for this bacterium were not widely available. A new test should help overcome this problem. The best antibiotic for chlamydia is a tetracycline in adults and an erythromycin in infants and children. Penicillin, often used to treat gonorrhea, is not effective. As with most sexually transmitted diseases, all sexual partners should be treated. Follow-Up

In response to the question on sarcoidosis, representatives of the American Lung Association of Maryland tell me that they've started a self-help support group for sarcoid patients and their families. They also have information on the disease and resources for further help. Write ALA of Maryland, 170 Rollins Ave., Rockville, Md. 20852, or call 881-6852. Jay Siwek, a family physician from Georgetown University, practices at the Fort Lincoln Family Medicine Center in Northeast Washington.