Abstract Background The German version of the Conners Adult ADHD Rating Scales (CAARS) has proven to show very high model fit in confirmative factor analyses with the established factors inattention/memory problems, hyperactivity/restlessness, impulsivity/emotional lability, and problems with self-concept in both large healthy control and ADHD patient samples. This study now presents data on the psychometric properties of the German CAARS-self-report (CAARS-S) and observer-report (CAARS-O) questionnaires. Methods CAARS-S/O and questions on sociodemographic variables were filled out by 466 patients with ADHD, 847 healthy control subjects that already participated in two prior studies, and a total of 896 observer data sets were available. Cronbach's-alpha was calculated to obtain internal reliability coefficients. Pearson correlations were performed to assess test-retest reliability, and concurrent, criterion, and discriminant validity. Receiver Operating Characteristics (ROC-analyses) were used to establish sensitivity and specificity for all subscales. Results Coefficient alphas ranged from .74 to .95, and test-retest reliability from .85 to .92 for the CAARS-S, and from .65 to .85 for the CAARS-O. All CAARS subscales, except problems with self-concept correlated significantly with the Barrett Impulsiveness Scale (BIS), but not with the Wender Utah Rating Scale (WURS). Criterion validity was established with ADHD subtype and diagnosis based on DSM-IV criteria. Sensitivity and specificity were high for all four subscales. Conclusion The reported results confirm our previous study and show that the German CAARS-S/O do indeed represent a reliable and cross-culturally valid measure of current ADHD symptoms in adults.