8-Azapurin-6-ones substituted in the 2-position by a phenyl group carrying in a meta-position a sulphamoyl substituent, and also a hydroxy, alkoxy or alkylthio substituent para with respect to the said sulphamoyl grouping, and pharmaceutically acceptable salts thereof, possess pharmacological properties useful in the treatment of allergic respiratory disorders, for example allergic bronchial asthma.