This thesis concerns the clinical epidemiology of HIV infection in Zimbabwean adolescents. Without treatment, there is a very high risk of death in the early years of life in HIV-infected infants. However, in recent years increasing numbers of adolescents have been presenting to health care services with symptomatic HIV infection and with features suggesting longstanding disease. Population-based surveys in Southern Africa have shown HIV prevalence rates among older children and adolescents to be much higher than would be anticipated if HIV-infants were not surviving early childhood. The burden and spectrum of HIV-associated morbidity among adolescents was investigated with two studies at secondary and primary care level, respectively. The main finding was of an extremely high prevalence of HIV infection at both levels of the health system, with HIV infection being the single most common cause of hospital admission and death among adolescents. Mother-to-child transmission was the most likely source of HIV infection in the majority, suggesting a substantial epidemic of older survivors of vertical HIV infection. Other countries with severe HIV epidemics may be experiencing a similar trend as their HIV epidemics mature. The lack of awareness of the possibility of survival to older childhood and adolescence with maternally-acquired, untreated HIV infection results in many missed opportunities for diagnosis, with HIV infection frequently not diagnosed until presentation with a severe HIV-related illness. The median CD4 count in 3 HIV-infected adolescents in primary care was 350cells/µl compared to a median CD4 count of 151cells/µl among hospitalised adolescents, suggesting that HIV testing in primary care identifies HIV-infected adolescents at an earlier stage of infection. Provider-initiated HIV testing and counselling in primary care was highly acceptable to adolescents and guardians. Provision of care has been adversely affected by under-appreciation of the numbers of surviving adolescents living with HIV, and the special needs of this age-group have not been distinguished from those of younger children. Young people who have acquired HIV perinatally are stigmatised by society who assume they must have acquired it through "bad" behaviour themselves, since it is not widely appreciated that long-term survival following vertical infection is possible. Immediate priorities are earlier diagnosis of HIV infection and improved management of HIV-infected adolescents. Possible areas of intervention are discussed in the final chapter. Similar studies are needed in neighbouring countries to investigate the generalisability of these findings.