The aim of this study was to evaluate the underreporting of some infectious diseases in the pediatric population in the Local Health Unit of Florence in the period 15.09.1997-14.09.1998. Data from the current notification system and from an a hoc sentinel network were used. Nine family pediatricians voluntarily participated in the sentinel network, notifying all cases of measles, mumps, pertussis, rubella, scarlet fever and chickenpox diagnosed in the population of children under their professional responsibility, in the period of the study. Chickenpox was the most frequent disease (2,043 cases equal to 73.5% of total notifications). The notification rate for chickenpox obtained with the sentinel network was 41.6 per 1,000 children, meanwhile the notification rate obtained with the current notification system was 23.7 per 1,000. The notification rate for scarlet fever was 24.1 per 1,000 with the sentinel network and 6.0 per 1000 with the current notification system. The underreporting for the two diseases was respectively 43% and 75%. Voluntary participation of physicians in sentinel network guarantees data of good quality, making these networks very useful tools for the epidemiologic evaluation of infectious diseases with benign prognosis.