ObjectiveThere are several reported cases of vertically infected children presenting with advanced HIV infection in the UK. The children of women with HIV infection are at increased risk of being infected. There are few data available on the number of such children that are yet to be tested for HIV. This study looked at the HIV testing status of children whose mothers attend HIV services at three south-west London clinics. MethodsCase notes of women attending the clinics from 1 January to 30 June 2009 were reviewed. When data were incomplete, women were prospectively interviewed. ResultsCase notes of 605 women were reviewed; 478 women had 1107 children. The majority of women (386; 81%) were of Black African ethnicity. Sixty-one per cent (675 of 1107) of the children were known to have been tested for HIV. The children resident abroad were more likely to be untested compared with those resident in the UK; 186 of 255 (73%) vs. 246 of 852 (29%). A quarter (106 of 432) of the untested children were 18 years old; 49 (46%) of these were resident in the UK. The most common reason given by the mothers for not testing was a perceived 'unlikely risk'. ConclusionsA significant number of children at risk of vertically transmitted HIV infection, including 49 children 18 years and resident in the UK, were identified through this study. The mothers are being encouraged to have these children tested and a multidisciplinary team involving adult and paediatric HIV healthcare professionals has been set up to negotiate and facilitate testing.