Objective: To assess the effectiveness of a home based early intervention on children's body mass index (BMI) at age 2. Design: Randomised controlled trial. Setting: The Healthy Beginnings Trial was conducted in socially and economically disadvantaged areas of Sydney, Australia, during 2007-10. Participants: 667 first time mothers and their infants. Intervention: Eight home visits from specially trained community nurses delivering a staged home based intervention, one in the antenatal period, and seven at 1, 3, 5, 9, 12, 18 and 24 months after birth. Timing of the visits was designed to coincide with early childhood developmental milestones. Main outcome measures: The primary outcome was children's BMI (the healthy BMI ranges for children aged 2 are 14.12-18.41 for boys and 13.90-18.02 for girls). Secondary outcomes included infant feeding practices and TV viewing time when children were aged 2, according to a modified research protocol. The data collectors and data entry staff were blinded to treatment allocation, but the participating mothers were not blinded. Results: 497 mothers and their children (75%) completed the trial. An intention to treat analysis in all 667 participants recruited, and multiple imputation of BMI for the 170 lost to follow-up and the 14 missing, showed that mean BMI was significantly lower in the intervention group (16.53) than in the control group (16.82), with a difference of 0.29 (95% confidence interval -0.55 to -0.02; P=0.04). Conclusions: The home based early intervention delivered by trained community nurses was effective in reducing mean BMI for children at age 2. Trial registration: Australian Clinical Trial Registry No 12607000168459.