Q. What is the difference between rheumatoid arthritis and osteoarthritis? I have pain in several of my joints, and I am curious to know what kind of arthritis I have. Will X-rays tell the difference, or do I need blood tests? I imagine that there are different treatments depending on the kind of arthritis one has.

A. There are dozens of types of arthritis besides rheumatoid arthritis and osteoarthritis. Some others include gout and infections such as Lyme disease and lupus. The first step is to see which kind of arthritis you have. And you're right that treatment varies depending on which kind it is.

Osteoarthritis is the most common type of arthritis. It is also called degenerative joint disease. In most cases, the cause is unknown. In others, it stems from some underlying factor such as prior injury to the joint (as a football player may have had). The rate of osteoarthritis rises with age. It is more common in women, and it also runs in some families. Being overweight makes you more likely to have the condition, especially in the knees.

Doctors diagnose osteoarthritis by the pattern of pain, the characteristic joints involved and their typical appearance on X-rays. The joints most often affected are in the fingers, thumbs, hips, knees and spine. Initially, the pain is made worse with movement and relieved by rest. The affected joint may be a little stiff when you first get out of bed or after not using it for a while.

Rheumatoid arthritis is a less common but potentially more serious condition. Its cause is unknown, but it seems to stem from something making your immune system attack your joints along with other parts of your body.

This type of arthritis typically affects at least three joints at the same time. These include the joints in the fingers, wrists, elbows, knees, ankles and feet. Joint stiffness in the morning, lasting an hour or more, is a prominent feature. Rheumatoid arthritis tends to be symmetrical, meaning that it affects similar joints on opposite sides of the body.

In addition to swelling, pain and joint stiffness, this type of arthritis can also cause little bumps or nodules under the skin, known as rheumatoid nodules. Many people with rheumatoid arthritis have a positive blood test for what's known as rheumatoid factor. The condition also produces a characteristic appearance on X-rays of the hands.

As for treatment, osteoarthritis responds to resting the affected joint, weight loss if overweight and physical therapy including ice, heat and exercises to strengthen the muscles around the joint. For severe hip or knee pain, using a cane can help.

Medicines don't cure osteoarthritis, but they can help relieve pain. Start with nonprescription medicines such as acetaminophen (Tylenol and other brands). Capsaicin cream--available without a prescription--can decrease pain and tenderness. Non-steroidal anti-inflamatory drugs, or NSAIDs, such as ibuprofen, are pain relievers that come in prescription and nonprescription strengths. For severe joint pain and swelling, an occasional injection of a steroid medicine can help.

If you have severe osteoarthritis of the knee, you can try an injection of a substance that mimics normal joint fluid. The idea is to cushion your knee, thereby reducing pain. The two new products for this series of weekly injections are Hyalgan and Synvisc. If all else fails and your osteoarthritis is really disabling, you can have surgery to do a total joint replacement, such as for your thumb, knee or hip.

For rheumatoid arthritis, some treatments are similar and include NSAIDs and physical therapy. But the main difference is the aggressive use of powerful drugs that try to halt the progress of this potentially crippling disease. These drugs include oral steroids such as prednisone, methotrexate (an anti-cancer drug) and other drugs that suppress the immune system. Other medicines used for rheumatoid arthritis include hydroxychloroquine, sulfasalazine, gold compounds and penicillamine.

Some new research shows that certain antibiotics such as minocycline and doxycycline may also improve your symptoms. Whether this is because these antibiotics simply help control joint inflammation or they are combating some undetected bacterium responsible for the arthritis isn't clear.

For more information on the different forms of arthritis and treatments for them, contact the Arthritis Foundation, 1330 West Peachtree St., Atlanta, GA 30309; the group can be reached by telephone at 1-800-283-7800 and on the World Wide Web at http://www.arthritis.org. The Washington area chapter of the foundation is at 4455 Connecticut Ave. NW, Suite 300, Washington, DC 20008; its telephone number is 202-537-6800.

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.