Few studies have analyzed healthcare utilization before suicide among individuals with attention-deficit/hyperactivity disorder (ADHD). This study examined the pattern of healthcare utilization and comorbidities shortly before death among patients with ADHD who died by suicide and compared these data with those of living controls. This study used Taiwan’s National Health Insurance Research Database to identify patients with ADHD (N = 379,440) between January 1, 2001, and December 31, 2016. Subsequently, the researchers identified 159 suicide decedents by linking each patient with the National Mortality Database. By conducting a nested case–control study with risk-set sampling from the ADHD cohort, the researchers selected 20 age- and sex-matched controls (n = 3180) for each patient who died by suicide (cases). The researchers then applied conditional logistic regression to investigate differences in healthcare utilization as well as psychiatric and physical comorbidities between case patients and controls. Case patients had higher healthcare utilization within 3 months before suicide, particularly in the psychiatry, emergency, internal medicine, neurosurgery, and plastic surgery departments. These patients also had higher risks of psychiatric comorbidities, including schizophrenia, bipolar disorder, depressive disorder, and sleep disorder, as well as physical comorbidities such as hypertension and other forms of heart disease. Among patients with ADHD, suicide decedents had increased healthcare utilization and higher risks of specific psychiatric and physical comorbidities than living controls. Thus, for suicide prevention among individuals with ADHD, suicide risk must be detected early and comorbidities should be adequately managed.