12 HIV-infected children were treated with a high dose of zidovudine (800/mg/m2/day) during a mean follow up period of 18 months. After starting treatment with zidovudine there was a subjective improvement and an increase in body weight, and a significant decline in serum HIV-antigenemia (p < 0.01) and serum IgG levels (p < 0.05). Zidovudine was generally well tolerated but there was serious hematological toxicity. In 9 patients (75%) dose reduction was necessary because of severe anemia (3 patients) or leucopenia (6 patients). Because of the high incidence of these side effects the starting dose of zidovudine was reduced to 600 mg/m2/day. These results confirm that zidovudine can safely be used to treat HIV-infected children. The main limitation is serious hematological toxicity which appears to be dose-related.