More than a third of patients on HIV medication in sub-Saharan Africa die or discontinue their treatment within two years of starting it, a survey shows.
The study found that many were too late taking up anti-retroviral (ARV) drugs, while for some it was impractical to travel to distant clinics.
The US researchers also found evidence that in cases where patients had to pay for ARVs, some stopped treatment.
But it showed success rates vary depending on the programme and country.
Details of the study by the Boston University School of Public Health are published online by the Public Library of Science.
The researchers looked at antiretroviral programmes for HIV patients in 13 sub-Saharan countries.
They found that two years on from the commencement of treatment, only 61.6% of all patients were still receiving medication.
The researchers say there are many reasons for the fall-out rate.
Many patients were too late in taking up ARVs in the first place and died within a few months of commencing treatment.
Other patients dropped out because of problems with accessing the drugs - they may live some distance, for example, from the clinic which provided the medication.
There was also evidence, the researchers say, of patients discontinuing treatment because of the cost of the drugs in those cases where patients were charged for their ARVs.
Boston University's Dr Christopher Gill says in many cases, taking the ARVs may take a back seat to more pressing daily needs.
"Receiving the drug itself is a major investment of a patient's time, so if you live 8 km from the nearest clinic and have to go there once a month and you don't have a ready means of transport it's a huge investment of your own time," he told the BBC.
"And if you're feeling well and you're worried about other things in terms of finding enough to eat or maintaining a job or finding a job I suppose if you were feeling well you might be tempted to see treatment as being a second-order priority.
For the director of the Association of People with Aids in Kenya, poverty, a lack of education and an element of stigma are all part of the problem.
"If people are not well educated on how to take the drugs, then some patients fall out, and if they do fall out then they develop resistance," Roland Gomol Lenya told the BBC.
"We find some people also suffer from stigma: in some workplaces, people are not able to carry their ARVs and take their ARVs freely at workplaces.
"I think there are also the issues of poverty, and the people who administer ARVs should also look at the poverty element, because sometimes because of poverty people are not able to access the centres.
"The centres are normally far away from where people live, and that has been a problem."
The study shows that retention rates between individual ARV programmes vary widely across Africa.
One programme in South Africa retained as many as 85% of their patients after two years while another in Uganda retained only 46% of patients after the same period of time.
© 2007 BBC