Background: There are some indications that maternal lifestyle during pregnancy (smoking and stress) contributes to symptoms of ADHD in children. We prospectively studied whether prenatal exposure to maternal smoking and/or stress is associated with ADHD symptoms and diagnostic criteria (according to DSM-IV) in 7-year-olds. Methods: Nulliparous Scandinavian women were consecutively recruited at their first prenatal health care visit and assessments of smoking and stress were collected at gestational weeks 10, 12, 20, 28, 32, and 36. Children were followed up at 7 years old. We obtained full data for 72% of the sample: ADHD symptoms were rated by 74% of mothers (n = 290) and 96% of eligible teachers (n = 208). Attrition analyses showed no differences on key variables between participants and non-participants at follow-up. Results: Results of multiple regression analyses showed prenatal exposure to smoking (beta = 0.16, p less than 0.01) and stress (beta = 0.18, p less than 0.01) were independently associated with later symptoms of ADHD. Results of logistic regression analyses showed that fulfillment of the diagnostic criteria for ADHD was related to exposure to prenatal stress (beta = 0.68, p less than 0.01) especially in boys. The results were not confounded by sociodemographic factors or birth outcomes. Conclusions: This study provides evidence that prenatal exposure to stress and smoking is independently associated with later symptoms of ADHD in human children, particularly for boys. Because stress and smoking are relatively common during pregnancy, and yet preventable, these results are of public health significance.