A baby girl in the US born with HIV and believed cured after very early treatment has now been found to still harbour the virus.
Tests last week on the four-year-old child from Mississippi indicate she is no longer in remission, say doctors.
She had appeared free of HIV as recently as March, without receiving treatment for nearly two years.
The news represents a setback for hopes that very early treatment of drugs may reverse permanent infection.
Dr Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, told US media the new results were "obviously disappointing" and had possible implications on an upcoming federal HIV study.
"We're going to take a good hard look at the study and see if it needs any modifications," he said.
By James Gallagher, Health editor, BBC News website
There was huge hope that the "Mississippi baby" would live a life free of the HIV.
Antiretroviral drugs can keep the virus in check in the bloodstream, but HIV has hiding places - known as reservoirs - in the gut and brain.
If treatment stops, then the virus emerges from its reservoirs and begins its assault afresh.
Doctors had hoped that starting drug treatment within hours of birth would prevent the reservoirs forming.
This seems not to have been the case.
This case was never going to lead to an HIV-cure for infected adults, who begin treatment months or years after infection.
The Mississippi baby has become a reminder of how difficult HIV is to defeat and how distant a cure really is.
The child, nicknamed the "Mississippi baby", did not receive any pre-natal HIV care.
Because of a greater risk of infection, she was started on a powerful HIV treatment just hours after labour.
She continued to receive treatment until 18 months old, when doctors could not locate her. When she returned 10 months later, no sign of infection was evident though her mother had not given her HIV medication in the interim.
Repeated tests showed no detectable HIV virus until last week. Doctors do not yet know why the virus re-emerged.
A second child with HIV was given early treatment just hours after birth in Los Angeles in April 2013.
Subsequent tests indicate she completely cleared the virus, but that child also received ongoing treatment.
Only one adult is currently believed to have been cured of HIV.
In 2007, Timothy Ray Brown received a bone marrow transplant from a donor with a rare genetic mutation that resists HIV. He has shown no signs of infection for more than five years.