Background: Attention deficit hyperactivity disorder (ADHD) is one of the most commonneurodevelopmental disorders in adolescence resulting in functional impairment that often persist toadulthood. Girls seem to be diagnosed with ADHD later in life than boys and more often have acomorbid major depressive disorder (MDD). It has been suggested that comorbidity with MDD caninterfere with early identification of ADHD.Aims: The aim of the study was to investigate the diagnostic delay for boys and girls with ADHD inchild and adolescent outpatient settings as well the interference of self-reported symptoms of MDD.Methods: Adolescent psychiatric outpatients were assessed with DSRS-A Screener (version of DepressionSelf-Rating Scale for Adolescents) at intake. Diagnostic delay for ADHD was estimated with Kaplan-Meiersurvival curve with separated analysis for boys/girls and for negative/positive MDD screening. Coxregression analysis was used to examine the association of sex and MDD screening.Results: Diagnostic delay for N = 252 (41% boys, mean age =15.23 years) was 3.5 years, 1.4 for boys and4.9 for girls (p = 0.004). Positive MDD screening was associated with prolonged diagnostic delay (from1.4 to 4.9 years p = 0.002), which in separated analyses was statistically significant only for girls from 1.8to 4.9 years (p = 0.018). In Cox regression male sex was associated with decreased diagnostic delay(p = 0.023) while positive MDD screening with prolonged diagnostic delay (p = 0.015).Conclusions: The diagnostic delay of ADHD was more than three times longer for girls. Co-occurringsymptoms of MDD prolonged diagnostic delay significantly only for girls.