Presenting the data at an international conference on Aids in Vienna, the WHO said that an estimated 5,2-million people were being treated for the Aidsvirus at the end of 2009 after an extra 1,2-million people started treatment during the year.
It described the increase - the largest in a single year - as "an extremely encouraging development" but called for more programmes to help patients receive treatment before the virus starts to make them very sick.
"Starting treatment earlier gives us an opportunity to enable people living with HIV to stay healthier and live longer," said Gottfried Hirnschall, WHO director of HIV/Aids.
An estimated 33,4-million people now live with HIV/Aids around the world. The United Nations (UN) says that 15-million people need Aids drugs, so the latest increase means that only just over one-third are getting them.
Earlier HIV treatment can prevent so-called "opportunistic infections" including tuberculosis (TB), which is the biggest killer of people with HIV.
The costs for HIV treatment in 2010 will be about $9-billion, according to the Joint UN Programme on HIV/Aids (UNAids).
The WHO says that deaths from TB could be reduced by as much as 90% if people with both HIV and TB started treatment earlier, when their immune systems have not been weakened too much by the virus.
The strength of a person's immune system is measured by CD4 cells and this measure is used by doctors to assess when HIV-infected patients should start receiving medicines.
The WHO previously recommended starting HIV treatment when a person's CD4 count dropped below 200 cells/mm3 but it now advises starting treatment earlier, at 350 cells/mm3 or below.
In rich nations, HIV patients begin treatment before their CD4 count drops significantly. As a result they are able to live longer and more normal lives despite having the incurable virus.
A lack of funds and of healthcare infrastructure in poorer nations, where the virus is more widespread, mean patients have to wait until they are very ill before they get access to drugs.
WHO estimates that HIV-related deaths could be reduced by 20% between 2010 and 2015 if the new treatment guidelines were broadly implemented. Evidence from scientific studies also shows that earlier treatment can be an effective way of preventing the virus from spreading.
"Because treatment reduces the level of virus in the body, it means HIV-positive people are less likely to pass the virus on to their partners," the WHO's Hirnschall said.
Bernhard Schwartlander, director for evidence, strategy and results at UNAids, said investing in earlier HIV treatment could save "millions of lives" and "millions of dollars" in future.
"People with weaker immune systems who come late for treatment require more complex and costly drugs and services than those who start treatment earlier," he said.