The natural history of HIV infection in perinatally-infected children is still poorly defined; more information about disease progression is needed for the design of clinical and therapeutic trials. According to the most recent reports, vertically-acquired infection seems to follow two different modes of progression, with the majority of patients expressing early signs of severe disease, whereas a subgroup of mildly affected infants has a longer survival, comparable to that previously reported for adults: the long term prognosis of this last patient group remains unknown. Current estimates of incubation periods in children are based on limited data, largely drawn from ill patients: hence subjects with a short incubation time are more likely to be observed. A case is described of a child born in 1982 to an asymptomatic intravenous drug abuser mother. HIV infection was detected in both mother and son only in 1984, but other risk factors for HIV infection of the infant (i.e. blood transfusions, sexual abuse) were excluded. The child had an uneventful neonatal period and childhood, with normal growth and development; up to now, at the age of 9 years 6 months, no clinical signs and symptoms possibly related to HIV infection became evident. Laboratory examinations showed only slight immunologic abnormalities (class P-1 B, CDC), such as increased serum immunoglobulin levels, reduction of CD4+/CD8+ ratio with no relevant decrease of absolute CD4+ lymphocyte count (1100/mm3); HIV-p24Ag and virus isolation were always negative.(ABSTRACT TRUNCATED AT 250 WORDS)