Clinical and epidemiologic studies have established that posttraumatic stress disorder (PTSD) is highly comorbid with other mental disorders. However, such studies have largely relied on adults' retrospective reports to ascertain comorbidity. The authors examined the developmental mental health histories of adults with PTSD using data on mental disorders assessed across the first 3 decades of life among members of the longitudinal Dunedin Multidisciplinary Health and Development Study; 100% of those diagnosed with past-year PTSD and 93.5% of those with lifetime PTSD at age 26 had met criteria for another mental disorder between ages 11 and 21. Most other mental disorders had first onsets by age 15. Of new cases of PTSD arising between ages 26 and 32, 96% had a prior mental disorder and 77% had been diagnosed by age 15. These data suggest PTSD almost always develops in the context of other mental disorders. Research on the etiology of PTSD may benefit from taking lifetime developmental patterns of comorbidity into consideration. Juvenile mental-disorder histories may help indicate which individuals are most likely to develop PTSD in populations at high risk of trauma exposure.