Purpose: The Conners Rating Scales are widely used in research and clinical practice for measuring attention deficit/hyperactivity disorder (ADHD) and associated problem behaviors, but country-specific norms are seldom collected. The current study presents the standardization of the Swedish Conners 3 V R Rating Scales. In addition, we compared the Swedish norms to those collected in the U.S. and Germany. Material and methods: The study included altogether 3496 ratings of children and adolescents aged 6-18 years from population-based samples. Results: The scores obtained for the Swedish Conners 3 V R showed satisfactory to excellent internal consistency for most subscales and excellent test-retest reliability. Across-informant correlations were modest. Crosscountry comparisons revealed that aggression symptoms rated by teachers and ADHD symptoms rated by parents differed between Sweden, Germany and the U.S. Executive functioning deficits also varied as a function of rater and country, with German and Swedish teachers reporting increasing behavior problems with age, whereas a decrease was observed in the U.S. For some subscales, the observed cross-cultural differences were large enough for a child to be classified as being within the normal range (t-score <60) in one country and within the clinical range (t-score > 70) in another country. Conclusion: The present study shows that the Swedish adaptation of the Conners 3 V R provides consistent and reproducible scores. However, across-informant ratings were only modest and significant cross-cultural differences in scoring were observed. This emphasizes the need for multi-informant assessment as well as for national norms for rating instruments commonly used within child and adolescent psychiatry research and clinical settings.