We have measured the P.E.F. of 542 infants with a portable spirometer and correlated the results with different spirometric and anthropometric parameters with a view to validate this functional respiratory exploration technique as a practical and cheap way of screening and observing such respiratory diseases as asthma, bronchitis and muscoviscidosis. The results perfectly correlate with the parameters and specially with the F.E. V1, both among normal subjects and those suffering from an obstructive or restrictive syndrome. Therefore the results validate this system for the screening and the clinical treatment of those diseases.