Introduction: Under current HIV transmission mechanisms operating in Vietnam, women are seen as victims of their male partners. Having multiple sexual partners is one of the well-known risk factors for HIV infection. However, little is known about women's risky sexual behaviour and their vulnerability to HIV in Vietnam. This study aims to explore association between early sexual initiation and the number of lifetime sexual partners in Vietnamese women. Although the Vietnamese culture is socially conservative in this area, identifying women's risky sexual behaviour is important for the protection of women at risk of HIV and other sexually transmitted diseases. Design: A total of 8,791 women, who reported having had sexual intercourse, were included in this analysis of data from the 2011 Multiple Indicator Cluster Survey in Vietnam. Data were collected using two-stage strata sampling, first at the national level and second across six geographical regions (n=8,791). Multivariable logistic regressions describe association between early initiation of a sexual activity and lifetime multiple sexual partners. Results: Early sexual intercourse was significantly associated with having lifetime multiple sexual partners. Women who were aged 19 or younger at first sexual intercourse were over five times more likely to have multiple sexual partners, compared with women whose first sexual intercourse was after marriage; aged 10-14 years (OR=5.9; 95% CI=1.9-18.8) at first intercourse; and aged 15-19 years (OR=5.4; 95% CI=4.0-7.2) at first intercourse. There was significant association with having multiple sexual partners for women of lower household wealth and urban residence, but the association with educational attainment was not strong. Conclusions: The study results call for health and education policies to encourage the postponement of early sexual activity in young Vietnamese women as protection against risky sexual behaviour later in life.