In 96 children with symptoms of asthma or allergic rhinoconjunctivitis, 503 provocation tests with some common allergens (animal danders, pollens, house dust) were performed and the results were compared with RAST, an in vitro method for determination of circulating reagins. The overall agreement between RAST and the provocation tests was 74%, with no significant difference between bronchial and rhinoconjunctival provocation tests. Equally good agreement was obtained with different allergens. In patients with pronounced allergy, as demonstrated by positive provocation tests using highly diluted allergens, RAST was positive in all cases. In patients with low-grade allergy or with negative provocation tests the agreement with RAST was not as good. RAST offers without doubt a most valuable in vitro method, especially in pediatric practice. However, provocation tests will have also in the future a definite place in the diagnosis of allergies.