HIV infection rates have fallen by a third in the worst hit regions of south India, research suggests.
Public awareness campaigns have promoted safe sex
The joint Indian and Canadian team say the figures show safe sex awareness campaigns can have a dramatic impact.
The Lancet study tracked HIV infection rates among young women attending pregnancy clinics, and young men attending sex disease clinics.
More than five million people are living with HIV in India, 75% of them in the southern states.
Male use of female sex workers is a main reason for the spread of HIV in these areas, which subsequently puts wives in a vulnerable position.
A leading Indian group working to increase Aids awareness welcomed the Lancet study but said it was "incomplete" and presented a "skewed" picture.
"I am glad about the report but my only concern is that without the death rates, the report would not be presenting the real picture," Anjali Gopalan of the Naz Foundation Trust told the BBC.
In recent years, the Indian government, the World Bank and other external agencies have aimed intervention and awareness programmes at the sex industry.
Researcher Professor Prabhat Jha, of the University of Toronto, said the latest study suggested these campaigns had confounded the sceptics who doubted whether they would work.
He said: "There have been many predictions, mostly based on guesswork, that India's Aids problem will explode - as it did in southern Africa - but we now have direct evidence of something positive.
It is claimed that Aids is chiefly spread among sex workers
"The good news is that HIV in young adults appears to be declining in the south - most likely or perhaps only due to males using sex workers less or using condoms more often when they do.
"The not-so-good news is that trends in the north remain uncertain and poorly studied."
The researchers studied HIV prevalence data from 294,050 women attending 216 antenatal clinics and 58,790 men attending 132 sexually transmitted infection (STI) clinics in the north and south from 2000 to 2004.
The study was led by Professor Rajesh Kumar, of the Postgraduate Institute of Medical Education and Research in Chandigarh.
He said the decline in HIV infection rates among women seemed to cut across class boundaries.
Professor Kumar said: "HIV remains a huge problem in India and we have to remain vigilant.
"We're not saying the epidemic is under control yet - we are saying that prevention efforts with high-risk groups thus far seem to be having an effect."
Paul Arora, of the Centre for Global Health Research, said: "A key implication of the study is the need to scale up highly effective prevention efforts in the north, especially in hot-spot urban and rural districts - not just for female sex workers but also for men having sex with men."
Last year India's Health Minister Anbumani Ramadoss defended the country's official HIV figures after UNAids chief Peter Piot said they were wrong.
India's figures showed just 28,000 new HIV infections in 2004 - down from 520,000 in 2003. Mr Ramadoss said they had been verified by the World Health Organisation.