BACKGROUND: The number of new diagnoses of HIV infection is rising in the northwestern hemisphere and it is becoming increasingly important to understand the mechanisms behind this trend. OBJECTIVE: To evaluate whether reported unsafe sexual behaviour among HIV- infected individuals is changing over time. DESIGN: Participants in the Swiss HIV Cohort Study were asked about their sexual practices every 6 months for 3 years during regular follow-up of the cohort beginning on 1 April 2000. METHODS:: Logistic regression models were fit using generalized estimating equations assuming a constant correlation between responses from the same individual. RESULTS: At least one sexual behaviour questionnaire was obtained for 6545 HIV-infected individuals and the median number of questionnaires completed per individual was five. There was no evidence of an increase in reported unsafe sex over time in this population [odds ratio (OR), 1.0; 95% confidence interval (CI), 0.96-1.05]. Females (OR, 1.38; 95% CI, 1.19-1.60), 15-30 year olds (OR, 1.26; 95% CI, 1.09-1.47), those with HIV-positive partners (OR, 12.58; 95% CI, 10.84-14.07) and those with occasional partners (OR, 3.25; 95% CI, 2.87-3.67) were more likely to report unsafe sex. There was no evidence of a response bias over time, but individuals were less willing to leave questions about their sexual behaviour unanswered or ambiguous (OR, 0.93; 95% CI, 0.90-0.97). CONCLUSIONS: There was no evidence of a trend in unsafe sex behaviour over time. However, several subgroups were identified as being more likely to report unsafe sex and should be targeted for specific interventions.