The present study investigated regional differences in response behaviour for the Patient Health Quetionnaire-9. We tested for measurement invariance and differential item and test functioning between formerly divided East- and West-Germany: the former German Democratic Republic and Federal Republic of Germany. Diverging socialization experiences in socialist versus capitalist and collectivist versus individualist systems may affect culturally sensitive assessments of mental health. To test this empirically, we used factor analytic and item-response-theoretic frameworks, differentiating between East- and West-Germans by birthplace and current residence based on several representative samples of the German general population (n = 3 802). Across all survey, we discovered slightly higher depression sum scores for East- versus West-Germans. The majority of items did not display differential item functioning-with a crucial exception in the assessment of self-harm tendencies. The scale scores were largely invariant exhibiting only small amounts of differential test functioning. Nonetheless, they made up on average about a quarter of the observed group differences in terms of effect magnitude. We explore possible causes and discuss explanations for the item-level differences. Overall, analyses of East- and West-German depressive symptom developments in the wake of reunification are feasible and statistically grounded.