Abstract Background: Sexual transmission continues to be the primary mode of human immunodeficiency virus (HIV) infection in Western Europe. We aimed to describe predictors of unsafe sex and reasons given for such behaviour. Methods: We performed a survey examining sexual risk behaviours and reasons for unsafe sex in a nationwide cohort of adult Danish HIV-1-positive patients. Differences in characteristics between those who practiced safe and unsafe sex were estimated by binary logistic regression. The fraction with detectable viral load was determined in the 2 groups, and reasons for unsafe sex were evaluated. Results: Of 812 eligible patients, a total of 275 (34%) had engaged in unsafe sex with an HIV-negative partner or a partner with unknown HIV status in the previous year. On multivariate analysis, men who have sex with men (MSM) was the only statistically significant risk factor associated with unsafe sex (odds ratio 3.24, 95% confidence interval 1.72-6.12). The main reason for practicing unsafe sex was that the partner did not wish to use a condom (53%). Conclusions: A high proportion of HIV-positive patients engage in unsafe sex, especially MSM. The reasons for unsafe sex are primarily linked to negotiation issues concerning condom use, including assumptions about the sexual partner's intent.