The effects of any drug, the way a drug enters the bloodstream, the way it is used by the body, and the way the body eliminates the drug may be very different in children compared with adults. The largest group of children who have AIDS are those who are less than 2 years of age. AIDS is often first identified in infants who are about 6 months old. Studies of AZT show that it might be useful in the treatment of AIDS. Thus it is important to study the effects of the drug in children.