The National Institute of Mental Health Diagnostic Interview Schedule,23 revised according to the DSM-III-R, was used. Panic attack is defined in the DSM-III-R as an attack in which at least 4 of the following 13 symptoms are experienced: shortness of breath, dizziness, palpitation, trembling, sweating, choking, nausea, depersonalization, numbness, flushes or chills, chest pain, fear of dying, and fear of going crazy. Panic disorder is defined as the occurrence of either 4 attacks within a 4-week period or 1 or more attacks followed by a period of at least 1 month of persistent fear of having another attack. The Diagnostic Interview Schedule elicits information on the age at which the first DSM-III-R panic attack occurred. Age at first panic attack is also used to define age of onset of panic disorder in the subset meeting the DSM-III-R criteria for the disorder. The onset of daily smoking is defined as the age at which the respondent first smoked daily for 1 month or longer. Lifetime history of panic attacks, psychiatric disorders, and smoking was measured at baseline, and interval history, covering the period since the last assessment, was gathered at each follow-up interview. Data from baseline and follow-up interviews were combined, providing lifetime data up to age 35 years, the upper age limit of the respondents at the last interview.