This study examined the prenatal, postnatal and demographic predictors of parent-reported attention-deficit/hyperactivity disorder (ADHD) in an Australian population-based sample. Participants were families participating in the Longitudinal Study of Australian Children. There were approximately even numbers of males (51%) and females (49%) in the sample. Predictors of parent-reported ADHD status at Wave 2 (children aged 6–7 years) which were measured at Wave 1 (children aged 4–5 years) included cigarette smoking and alcohol use during pregnancy (prenatal factors); maternal postnatal depression, intensive care at birth, birth weight, and gestation (postnatal factors); and child gender, primary caregiver education, income, family composition, and maternal age at childbirth (socio-demographic factors). We found that male gender, cigarette smoking during pregnancy, and maternal postnatal depression were the only significant predictors (at the 5% level) of ADHD in the adjusted analysis (N = 3,474). Our results are consistent with previous findings that male gender and cigarette smoking during pregnancy are risk factors for ADHD. In addition, we found that postnatal depression was predictive of parent-reported ADHD.