Children living with HIV often display delayed motor performance owing to HIV infection of the central nervous system, the effects of opportunistic infections and, indirectly, owing to their social environments. Although these problems have been well documented, the impact of the virus on the development of South African children is less well known. The aim of this study was to document the motor performance of a group of HIV infected children in Cape Town, South Africa. The Bayley Scales of Infant Development Second Edition were administered to 51 HIV infected children [mean age 15.8 months (SD=7.5, range 6.2-31.7 months)] of whom 34 were receiving antiretroviral therapy. Their performance was compared with an age-matched reference sample (n=35), whose HIV status was unknown. The HIV infected sample and the age-matched sample were comparable with regard to caregiver's level of education (P=0.42), employment status (P=0.35) and income (P=0.28). However, the HIV infected sample had significantly more hospital admissions (P less than 0.01), their caregivers were mostly single (P=0.04) and most lived in formal houses (P less than 0.01). The prevalence of significant motor delay was 66.7% in the HIV infected sample compared with 5.7% in the age-matched sample. As expected, the performance of the HIV infected sample was significantly poorer than the age-matched sample. However, the extent of the delay is a cause of concern as so many children presented with significant motor delay. Unexpectedly, a significant number of healthy children also displayed delayed performance. It is recommended that the developmental performance of HIV infected children be monitored over an extended period to determine whether the developmental delay can be reduced with treatment. In the interim, there is a need to provide stimulation and treatment to the large number of children who are developmentally delayed as a result of HIV infection, including those uninfected children in the community who are at risk owing to their socio-economic status.