My son Jonathan, 12 and asthmatic, started using POWERbreathe about a year ago. After using it regularly for about 3 months his peak flow increased from 400 to 420!! (measures from peak flow meter). He still uses POWERbreathe now, less regularly, and his peak flow has been steady and high. We were so happy with the results that we bought one for his younger cousin, also asthmatic!