Objectives: To identify the biological, socio-demographic, work-related and lifestyle determinants of physical activity in young adult women. Design: Prospective cohort study. Methods: Self-reported data from 11,695 participants (aged 22-27 years in 2000) in the Australian Longitudinal Study on Women's Health were collected over 9 years in 2000, 2003, 2006 and 2009. Generalised Estimating Equations were used to examine univariable and multivariable associations of body mass index, country of birth, area of residence, education, marital status, number of children, occupational status, working hours, smoking, alcohol intake, and stress with physical activity status (active, ≥600. MET·min/week; or inactive, <600. MET·min/week, consistent with public health guidelines). Results: All variables were significantly associated with physical activity in univariable models. In the multivariable model, the lowest odds of being active (compared with the relevant reference categories) were for women who: were born in Asia (OR. =. 0.53), had less than 12 years of education (OR. =. 0.79), were married (OR. =. 0.66) or in a de facto relationship (OR. =. 0.79), had at least one child (OR ranging from 0.67 to 0.69), and were classified as non (OR. =. 0.66) or rare drinkers (OR. =. 0.79). Conclusions: These results are among the first to confirm the biological, socio-demographic, work-related and lifestyle determinants of physical activity in women in their twenties and early thirties. These findings may be used to inform and improve the development of strategies, and to identify target groups most in need of intervention effort.