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To determine whether mode of delivery or the use of maternal or neonatal antiretroviral prophylaxis influence the age when HIV-1 can first be detected in infected infants, particularly the probability of detection at birth.In a collaboration between four multicentre studies, data on 422 HIV-1 infected infants who were assessed by HIV-1 DNA PCR or cell culture before 14 days of age were analysed. Weibull mixture models were used to estimate the cumulative proportion of infants with detectable levels of HIV-1 according to use of maternal/neonatal antiretroviral therapy (mainly zidovudine monotherapy) and mode of delivery.HIV-1 was detected in 162 infants (38%) when they were first tested, at a median age of 2 days. At birth, it was estimated that 36% [95% confidence interval (CI), 31–41%] of infants have levels of virus that can be detected by DNA PCR or cell culture. This percentage was not associated with either mode of delivery (35% for vaginal delivery versus 40% for cesarean section delivery;P = 0.4) or the use of maternal or neonatal antiretroviral prophylaxis. Among infants with undetectable levels of HIV-1 at birth, the median time to viral detectability was estimated to be 14.8 days (95% CI, 12.9–16.8 days). This time was increased by 15% (95% CI, −11 to 48%;P = 0.3) among infants who were exposed to antiretroviral therapy postnatally compared with infants who were not exposed. No effect was observed for mode of delivery.The outcome of an early virological test for HIV-1 is thought to be related directly to the timing of transmission and cesarean section delivery primarily reduces the risk of intrapartum transmission. The absence of an association between mode of delivery and viral detectability at birth was therefore unexpected. There was no evidence that foetal or neonatal exposure to prophylactic zidovudine delays substantially the diagnosis of infection, although this cannot be inferred for combination antiretroviral therapy.