Abstract OBJECTIVE: With the current increase in sexually transmitted infections in industrialized countries, we assessed the characteristics and plasma viral load of HIV-1-infected patients reporting sexual behaviour at risk for HIV transmission (SBR). DESIGN: The study population consisted of 223 patients with primary HIV-1 infection who were enrolled in the French PRIMO cohort between 1996 and 2001 and who had at least 3 months of follow-up. Patients were interviewed on condom use at each visit according to the partner (gender, steady versus casual nature, and HIV serostatus). SBR was defined as unprotected sex with partners of unknown or negative HIV serostatus. RESULTS: Sixty-one SBR were reported by 43 patients. SBR with casual partners increased from 5.1% in 1998 to 21.1% in 2001-2002, after a fall between 1997 and 1998. Reporting of SBR was more frequent among patients with casual partners, those with asymptomatic or briefly (< or = 15 days) symptomatic primary infection, and those who had developed clinical lipodystrophy or signs of anxiety/depression. Eighty-six per cent of patients reporting SBR had previously initiated highly active antiretroviral therapy (HAART); plasma viral load was above the detection limit (200/500 copies/ml) in 41% of visits reporting an SBR. Viral load was similar in patients reporting SBR and other patients, suggesting that the patient's knowledge of his/her response to HAART was not a major determinant of subsequent SBR. CONCLUSION: Our results confirm the recent increase in unsafe sex observed among HIV-infected individuals in industrialized countries. Levels of viral load of these individuals raise concern about the potential for re-emerging HIV epidemics.