Background: Since developmental psychologists are interested in explaining age and gender differences in depression across adolescence, it is important to investigate to what extent these observed differences can be attributed to measurement bias. Measurement bias may arise when the phenomenology of depression varies with age or gender, i.e., when younger versus older adolescents or girls versus boys differ in the way depression is experienced or expressed. Methods: The Children's Depression Inventory (CDI) was administered to a large school population (N = 4048) aged 8-17 years. A 4-factor model was selected by means of factor analyses for ordered categorical measures. For each of the four factor scales measurement invariance with respect to gender and age (late childhood, early and middle adolescence) was tested using item response theory analyses. Subsequently, to examine which items contributed to measurement bias, all items were studied for differential item functioning (DIF). Finally, it was investigated how developmental patterns changed if measurement biases were accounted for. Results: For each of the factors Self-Deprecation, Dysphoria, School Problems, and Social Problems measurement bias with respect to both gender and age was found and many items showed DIF. Developmental patterns changed profoundly when measurement bias was taken into account. The CDI seemed to particularly overestimate depression in late childhood, and underestimate depression in middle adolescent boys. Conclusions: For scientific as well as clinical use of the CDI, measurement bias with respect to gender and age should be accounted for. (Contains 4 tables, 1 figure and 5 footnotes.)