The criteria for asthma ('ever having asthma') were met by a "yes" response to all of the following questions (1) Has your child ever had an attack of wheezing that has caused him/her to be short of breath? (2) Has your child had such an attack twice or more? (3) Has a physician ever informed you that your child has asthma? and (4) If so, at that time, did your child have shortness of breath with wheezing? The subjects who had never had asthma, including the children who answer one, two, or three of the questions to define asthma, were used as controls.