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Background The World Health Organisation (WHO) advocates early initiation of HIV treatment as a prevention strategy among people living with HIV. There is strong evidence for the effectiveness of antiretroviral therapy (ART) as a preventive tool for HIV transmission. We aimed to determine the sexual behaviour of HIV outpatients and assess if it reflects the current preventive strategy for HIV transmission. Methods We conducted a cross-sectional study among adult (aged at least 18 years) patients with confirmed HIV diagnosis, and aware of their diagnosis, attending HIV outpatient care in Kenya. Data were gathered through self-report (using validated questionnaires) and file extraction. Multivariate logistic regression assessed the association between sexual risk taking behaviour controlling for gender, HIV clinical stage, HIV treatment status, Tuberculosis (TB) treatment status, and CD4 count. Results We recruited n = 400 participants (n = 280[70%] female gender). The mean age was 39.4 (SD = 9.9) years. The mean CD4 count was 393.7 (SD = 238.2) and ranged from 2 to 1470 cells/mm3. N = 61 (15.64%) were on TB treatment. The majority (n = 366, 91.5%) were on ART. Just over half (n = 202, 50.5%) reported having a sexual partner. Of these n = 33 (16.1%) reported having unprotected sexual intercourse with a person of unknown HIV status in the previous 3 months. Multivariate analysis showed that participants not on ART (HIV treatment) were more likely to report unprotected sexual intercourse compared to those who were on ART (odds ratio .25, 95% CI .09 to .69; P = 0.007). Participants at early stage of HIV infection (stages 1/2) were more likely to report unprotected sexual intercourse compared to participants at advanced HIV infection (stages 3/4) (odds ratio .34, 95% CI .13 to .92; P = 0.035). Males participants were more likely to be involved in sexual risk taking behaviours compared to female participants (odds ratio .36, 95% CI .16 to .82; P = 0.015). TB treatment status, and CD4 count were not significantly associated with sexual risk taking. Conclusion Participants not on ART have more unprotected sexual intercourse than those who are on ART. This calls for the need to scale up coverage and early ART initiation in order to reduce transmission of HIV.