The authors assessed the relation between personality and mental disorder in a representative birth cohort of 897 men and women. Personality was assessed at age 18 with the Multidimensional Personality Questionnaire (MPQ; A. Tellegen, 1982), and 4 types of mental disorder (affective, anxiety, substance dependence, and conduct disorder) were assessed at ages 15, 18, and 21, using age-appropriate standardized diagnostic interviews. All disorder groups had MPQ profiles that were very different from those of controls. When comorbid cases were excluded, fewer significant differences between diagnosed cases and controls remained. Relations between personality and mental disorder were not affected by the measurement of disorder as continuous versus discrete, gender, or the age at which disorder was diagnosed. Relations between personality and mental disorders appear to be robust, and individual personality differences may be particularly relevant to understanding the most severe (comorbid) expressions of psychopathology.