BACKGROUND: Previous research has linked attention deficit hyperactivity disorder (ADHD) with an increased risk of all-cause mortality, primarily owing to unnatural causes such as accidents and suicides. This increase may be attributable to the co-occurrence of major psychiatric disorders, including schizophrenia (SCZ), bipolar disorder (BD), major depressive disorder (MDD), autism spectrum disorder (ASD), anxiety disorders, substance use disorders (SUDs), and personality disorders (PDs). This study examined the all-cause and specific-cause mortality rates in individuals with ADHD and the influence of psychiatric comorbidities. METHODS: Between 2003 and 2017, 1.17 million individuals were enrolled in the study, of which 233,886 received a diagnosis of ADHD from the Taiwan's National Health Insurance Research Database. A 1:4 sex- and birth year-matched control group without ADHD was also included. Hazard ratios (HRs) for mortality rates were estimated between groups after adjusting for demographic data. RESULTS: During the follow-up period, 781 individuals with ADHD died. The HR for all-cause mortality was 1.45 (95% confidence interval [CI]: 1.30-1.61), largely owing to unnatural causes, particularly suicide. Suicide rates were particularly high in individuals with ADHD and psychiatric comorbidities: the HRs for suicide were 47.06 in ADHD with SUDs (95% CI: 6.12-361.99), 32.02 in ADHD with SCZ (7.99-128.29), 23.60 in ADHD with PDs (7.27-76.66), 10.11 in ADHD with anxiety disorders (5.74-17.82), 9.30 in ADHD with BD (4.48-19.33), 8.36 in ADHD with MDD (5.66-12.35), and 6.42 in ADHD with ASD (1.83-22.53) relative to ADHD only. DISCUSSION: ADHD was associated with increased mortality rates, primarily owing to suicide. The presence of major psychiatric comorbidities was associated with a further increase in suicide mortality risk.