The overall objective of this study was to determine whether smoking during pregnancy is related to psychiatric disorders in 4-year-olds while controlling for a wide range of potential confounding variables (i.e. parental anxiety, depression, personality disorders, drug abuse, and socio-economic characteristics). Parents of a community sample of 4-year-olds (N = 995) residing in the city of Trondheim, Norway were interviewed using the Preschool Age Psychiatric Assessment, which includes information on prenatal smoking. After adjusting for potential confounding variables using the propensity score, smoking during pregnancy was found to increase the odds for attention-deficit/hyperactivity disorder (ADHD) OR = 2.59 (CI 1.5-4.34, p < 0.001), oppositional defiant disorder (ODD) OR = 2.69 (CI 1.84-3.91, p = 0.02) and comorbid OR = 2.55 (CI 1.24-5.23, p < 0.001). Prenatal smoking during pregnancy is associated with an increased risk for symptoms of ADHD and ODD independently of each other, in 4-year-olds.