The provocative dose of inhaled methacholine required to cause a 20% drop in the forced expiratory volume in 1 sec was evaluated in two selected pediatric populations. On the basis of a standardized respiratory questionnaire, 165 individuals 5 to 21 yr of age were identified. Included were 110 normal nonatopic individuals and 55 current asthmatic subjects. Methacholine inhalation challenges were performed by use of a standard inhalation procedure. Fifty-four (98.1%) of the asthmatic subjects responded to methacholine with a 20% drop in the forced expiratory volume in 1 sec. Seventy (63.1%) of the normal individuals did not respond to methacholine. The specificity and sensitivity of the methacholine challenge was best obtained at a provocative dose of 100 breath units of methacholine.