Eight children with acquired immunodeficiency syndrome (AIDS), aged four months to 12 years, were treated with zidovudine (azidothymidine) 100 mg/m2 intravenously every six hours for 14 days, followed by oral zidovudine at the same dose for a total of six months. Of the eight, six were infected at birth and two were contaminated by blood transfusion at ages eight and nine years, respectively; seven of the eight showed specific neurologic impairment consisting of encephalopathy (six children) and myelopathy (one child). In two children, a dramatic improvement of clinical status occurred, including neurologic progress in one; in three, the improvement was dissociate or transient and in the other three, no modification was observed. A marked increase of total lymphocyte and CD4(+) cell counts occurred in four children but was transient without modification of in vitro antigen-induced lymphocyte proliferation; p24 human immunodeficiency virus serum antigens were detected in seven of eight children, then transiently disappeared in all children during intravenous therapy but reappeared progressively during the oral regimen in all but one. Progressive modification of human immunodeficiency virus serology was noted in five children, mainly characterized by the finding of anti-core antibodies. The hematologic toxicity of zidovudine was comparable with that observed in adults. These preliminary results support the need for further studies in order to delineate the optimal regimen of zidovudine in children with AIDS.