Objectives: To investigate the developmental time frame of epilepsy onset on adult personality traits of neuroticism and extraversion and to consider their role in adjustment to intractable epilepsy. Design: Prospective, preoperative and postoperative survey of the psychological and psychosocial effects of intractable epilepsy and its surgical treatment. Data from the preoperative phase are reported. Setting: Comprehensive Epilepsy Program (CEP), Austin Health. Patients: Sixty adult patients with focal epilepsy undergoing inpatient monitoring. Groups of patients with epilepsy onset in different developmental periods were empirically derived and compared with each other and with normative personality data from 1571 cases. Main Outcome Measures: Scores on the Eysenck Personality Questionnaire Revised-Short Form; the Beck Depression Inventory-II; the State-Trait Anxiety Inventory (state form); and the Austin CEP Interview, a semistructured interview providing in-depth psychosocial assessment. Results: Patients with onset of epilepsy during the self-defining period of adolescence had higher neuroticism scores relative to normative data (95% confidence interval, 0.16 to 3.57) and other patients (-0.46 to -5.63). High neuroticism, particularly when accompanied by lower extraversion, predisposed to poor adjustment to intractable epilepsy as reflected by impaired mood (P < .01) and difficulties with family functioning (48% of patients). Conclusions: These data provide initial evidence that onset of chronic neurologic illness in adolescence influences the development of adult personality traits. We also found a relationship between personality and adjustment to chronic epilepsy. The findings are relevant to the provision of psychologically informed neurologic care.