Background Casual sex among travellers is common. It is unknown whether travellers use condoms differently with local versus western casual partners when visiting HIV endemic areas. We determined the number of casual sexual partners and consistency of condom use among Dutch long-term, non-expatriate travellers to (sub)tropical regions according to the ethnicity of their casual sexual partners, and estimated the incidence of HIV and syphilis on their return.

Methods A prospective mono-centre study of Dutch long-term travellers ≥ 18 years, attending the Public Health Service travel clinic in Amsterdam (2008–2011) and travelling to any (sub)tropical country for ≥ 12 and ≤ 52 weeks, was conducted. Travelers reported their travel purpose, duration, destinations(s), number and nature of sexual contacts while travelling: ethnicity, gender, partner type (steady/casual) and consistency of condom use with each partner. Analyses were conducted using Poisson regression (generalised estimating equations to account for multiple partnerships). Blood samples, taken before and after travel, were tested for HIV and Treponema-pallidum antibodies.

Results There were 552 respondents and 11671 person-weeks of follow up (median age:25 years, 36% male, median travel time:20 weeks, 45% for work/study). Post travel, 34%(n = 190/552) reported ≥ 1 casual sexual partner, men a median of 3 (range:1–8) and women 2 (range:1–7) partners. Of 462 casual sexual partnerships, 42%(n = 192) were with local partners at travel destination. Equally, 39% of partnerships with western and local partners were unprotected. Single travellers (IRR_{steady partner(ref)}:2.2.95% CI: 1.2–4.0) and those on holiday (IRR_{work/study (ref)}:1.9.95% CI: 1.2–3.0) had more unprotected casual sex. Partner’s ethnicity was not significant in predicting condom use. No HIV or syphilis seroconversions were recorded.

Conclusion Unprotected casual sex was common among Dutch long-term travellers, occasionally with multiple local partners in HIV-endemic regions. Single travellers and those travelling for holiday purposes were most at risk. These groups should be advised on the need for safe sex while abroad.