HIV may still be able to attack the brains of patients even if they are taking a powerful cocktail of drugs, claim researchers.
HIV drugs dramatically increase life expectancy
However, experts in the UK say that there is no evidence that patients on long-term treatment are falling prey to brain illnesses linked to HIV.
US researchers described, in the journal NeuroReport, finding signs of damage in patients on anti-HIV drugs.
It is not clear if this damage happened before or after treatment started.
Many antiretroviral drugs - designed to suppress levels of HIV in the body - do not work in the brain, because they cannot get past the "blood brain barrier", a filter designed to protect the organ from large, potentially toxic molecules.
This theoretically means there is more chance that HIV might be found at higher levels in the brains of patients, even though the virus has been reduced to undetectable levels in the rest of the body.
HIV causes neurological damage in some patients, leading to symptoms such as loss of concentration, mood changes, slowing of reaction time and even a form of dementia.
Before the advent of modern antiretroviral combinations, one in five of those with HIV developed dementia.
Since the therapy was introduced, the number of patients reporting these symptoms has fallen rapidly, just as the drugs have dramatically increased life expectancy.
The research team, from the University of California at San Francisco, tested the brain functioning of a small number of HIV patients, and compared these with test results from a similar number of people who were free from the virus.
Some of the HIV patients were on antiretroviral therapy (ARV) and some were not on these drugs.
As well as recording brainwaves during set tasks, other tests to assess mental agility, and MRI scans to check the size of different brain areas were carried out.
The team found that in a particular brainwave test, the contingent negative variation test - a measure of anticipation and preparation and the overall ability of someone to react to something by taking a physical action - showed the biggest differences.
Among the HIV patients, both those taking ARVs and those not taking them showed similar, poor, results.
Bad results in this test suggest - but do not prove - that there may be damage in a part of the brain called the basal ganglia.
The MRI scans revealed, however, that HIV patients, regardless of their therapy regime, had signs of possible damage to another part of the brain called the thalamus.
Other results were mixed - on several tests there were no differences between any of the groups.
On a few, those HIV patients not taking medication scored worse than both the other HIV patients and the healthy control subjects.
Dr Linda Chao, who led the study, said that the study results were potentially worrying.
"You see people on ARV medications and they seem fine," she said.
"But the take-home message is that antiviral medications might not be stopping brain damage. When we put patients' brains under closer scrutiny, we saw that they were affected."
However, a clear limitation of the study was that there were no results for each patient prior to the starting of ARV treatment.
This would provide more substantial evidence that the damage had occurred after the patient started on the drug regime, and not before.
Dr Michael Weiner, another of the study's authors, said: "What we don't know is whether or not these changes occurred some time ago, prior to effective treatment, or whether these changes represent ongoing injury."
In the UK, Dr Keith Alcorn, from National AidsMap, told BBC News Online that the study results did not offer compelling evidence that HIV could inflict damage to the brain even after the start of ARV treatment.
He said: "There is no evidence from any large study that people with HIV develop brain problems on a large scale after starting treatment.
"We would know if this was happening by now - these drugs have been around for a while now."
He said that one message of the study was actually encouraging.
While debate continued about the need to get anti-HIV drugs into the brain itself, the results suggested that in some cases, neurological problems might be improved simply by the overall effect of the drugs on the immune system.