Objective. To examine changes in cardiovascular risk factors and health-related behaviours in young Australian adults at a stage of transition from the family environment.Study design. Repeated surveys between 9 and 25 years of age in a community-based group that included 569 eighteen-year-olds and 600 twenty-five-year-olds.Results. There were significant increases (P < 0.001 for all variables) in body mass index (BMI) [men 2.5 kg/m(2) (2.0); women 1.7 kg/m(2) (2.9)], waist girth [men 7.6 cm (6.5); women 4.3 cm (7.2)], BP (systolic/diastolic) [men 5(12)/7(8) mm Hg; women 3(10)/6(7) mm Hg] and in total cholesterol [men 15% increase; women 9%]. The proportion of sedentary behaviour increased from 19% to 39% in men (P < 0.001) and from 40% to 41% (P = 0.801) in women. Cohabitation was associated with significantly greater increases in BMI, waist circumference, and total cholesterol, associated with dietary change in women and decreased physical activity in men. In mothers, waist girth increased by 8.0 cm (0.1) compared with 3.5 cm (0.6) in women without children (P = 0.003), and physical fitness decreased [-0.5 W/kg (0.4) vs. 1.2 W/kg (0.2), respectively; P = 0.001].Conclusion. Encouragement of a healthy lifestyle, particularly physical activity, should be a priority in this age group, particularly among newly cohabiting couples and in young mothers. (C) 2004 The Institute For Cancer Prevention and Elsevier Inc. All rights reserved.