In 2011, the city of São Paulo started investigations on lost opportunities for HIV mother-to-child transmission prevention on notified children born since 2000, infected by their mother’s HIV. Main office team, responsible for STI/HIV epidemiological surveillance, controls the new mother-to-child HIV transmission notifications, sending each case to their home region. The local teams investigate the cases on diagnostic sites, following, notification and, if necessary, family or caregiver’s interviews. Data collected on mothers and children are registered on a database, improved and analysed on Microsoft Excel. From 2011 to 2016, 188 children born between 2000 and 2015 which were infected by mother’s HIV were investigated. From which, 44 mothers (23,4%) were diagnosed before pregnancy, 25 (13,3%) during pregnancy, 22 (11,7%) during labour, 68 (32,2%) after labour and 29 (15,4%) there was no information. Among the 188 investigated mothers, 117 (62,2%) attended to prenatal consults, 42 (22,4%) didn’t and there was no information on the other 29 (15,4%). 13 (13,4%) of the 97 women diagnosed after labour or without information did not attend to prenatal consults, 4 (4,1%) did it late, 2 (2,1%) denied to do it, on 2 of them (2,1%) the exams were not prescribed and 21 (21,6%) of them had negative result on the exam. Among those 97 women, 52 (53,6%) attended to prenatal consults while 21 (21,6%) didn’t, and the 24 remaining (24,8%) there’s no data. From the 52 that attended to prenatal consults, but had after labour diagnostic or without this information, 46 (88,5%) breastfed their children and 15 (28,8%) didn’t give their children the antiretroviral drugs properly. Whereas women may be infected at the end of pregnancy, making harder diagnostic or during breastfeeding, close follow-up of just delivered woman, with periodic HIV testing and the incentive to using condoms among breastfeeding and pregnant women is important. Moreover, women more vulnerable to HIV must be encouraged to do the tests, prenatal and postnatal.