QUESTION: Could you tell me about chondromalacia of the knee? What actually causes this painful disorder? What is the treatment? Will it get worse over the years? Is there a permanent cure? - E.E.D.

ANSWER: Chondromalacia patella is one of the more common knee disorders. It comes about from softening of the cartilage that cushions the kneecap against the leg bones when you bend or straighten the leg. Climbing stairs or prolonged sitting in one position causes the knee to ache. That has a name, "theater sign," referring to the need to sit still for long periods in a theater.Sometimes, an injury will cause it, perhaps one that knocks the kneecap a bit out of line. This unnatural positioning is enough to begin the cartilage crumbling process (malacia). Overuse of the legs without rest can be a cause. Often, no single cause can be found.

The future depends on what you do now. Early on, rest, anti-inflammatory drugs, even ice massages permit healing. If it is more serious, you may need crutches to permit unhampered healing. Surgery comes into play only rarely.

The future also depends on how well you restrengthen the leg muscles and restore flexibility following the enforced immobility. If everything goes well, chondromalacia need not give you trouble again.

QUESTION: I have been suffering with allergies (I assume). I have stuffiness, dry throat, cough, nasal voice and headaches all year round. Doctors have treated me with different medications, like Seldane, cortisone and antibiotics. X-rays show my sinuses are OK. Why the doctors haven't suggested skin testing for allergies I'll never know. Please comment on my miseries. They are getting the best of me. - D.K.

ANSWER: If you see an allergist, and if he believes your symptoms are allergy-caused, he will skin test you, I promise that. Not all allergies, I might add, are seasonal. Year-round allergy symptoms indicate year-round exposure, perhaps in the home or workplace.

QUESTION: I am writing for information about a problem that my 13-year-old grandson has. He sometimes will be sitting perfectly still, and his arms jerk and he makes a strange noise with his mouth. He even jerks in his sleep. His stepfather says it's a habit. I don't think it is that. Do you have any idea what it might be? It seems to be getting worse. - V.H.

ANSWER: The stepfather is wrong. The child needs to see a doctor.

Many children, perhaps three out of 100, have transitory tics that they outgrow. However, when jerking arms or legs, or facial grimaces are coupled with signs like hissing, barking or involuntary profanities, you have to think about Tourette's syndrome, a condition a lot more common than we used to think. As many as one in 200 children, most of them males, have it.

These tics begin before age 21 and last about a year, and there are medicines to control them. Not all may need treatment, such as children with infrequent episodes. The point is that all such children do need the attention of a professional, who can explain what is happening and what can be done.

It is not a habit, and, as you can imagine, Tourette's syndrome can be socially devastating for the child and those around him. Show this to the child's parents and have the problem investigated.

Endometriosis is one of the leading causes of female infertility and causes great pain and life disruption. Its causes and treatments are explained in Dr. Donohue's booklet No. 37. For a copy, send a long, stamped, self-addressed envelope and $2 to Dr. Donohue/No. 37, P.O. Box 830, Gibbstown, NJ 08027-9909.

Dr. Donohue welcomes reader mail but regrets that, due to the tremendous volume received daily, he is unable to answer individual letters. Readers' questions are incorporated in his column whenever possible.