The findings are based on serial surveys of 6500 men in Edinburgh and Glasgow, who visited a selection of gay bars in the two cities.
The men were questioned about their sexual behaviour as well as their attitudes to HIV infection and its treatment in 1996, 1999, and 2002.
Around two thirds of the respondents were 26 or older, and four out of 10 said they visited gay bars once or twice a week. The results showed that there was no significant change in behaviour between 1996 and 1999, but between 1999 and 2002, rates of unprotected anal sex rose 10%, as did levels of the activity with casual or multiple partners.
In 1996, almost 11% of those surveyed said they had unprotected anal sex with casual partners. By 2002, this figure was almost 19%.
The likelihood of unprotected sex with casual partners was greater among those men who said they visited gay bars frequently and those who agreed with the statement: "I am less worried about HIV infection now that treatments have improved."
Among those men who took an HIV test, only around one in five claimed that they "always" knew the HIV status of their casual partners.
While many researchers have reported that increasing optimism about the effectiveness of treatment has increased risky sexual behaviour among gay men, closer analysis of the findings showed that this did not fully explain the trends.
The increasing prevalence of unprotected anal sex with casual partners was actually higher among those who were not optimistic about HIV treatment.
The findings prompt the authors to call for renewed national efforts to spread the public health message about the need for HIV prevention to counteract "prevention fatigue."
Journal
Sexually Transmitted Infections