The minimum rate of underregistration of live births in the city of Piripiri, Piauí, between July 1, 1983 and June 30, 1984, is calculated. The lack of trustworthy data on underregistration of births is of the utmost importance in the area of Public Health, principally to those responsible for planning the Health Services. The method used to measure underregistration was the comparison of official data (Civil Registry Office) with unofficial data (Foundation SESP and the Catholic Church), the results of which show a very high minimum rate of underregistration--68.4%--as compared with the rates of other places in Brazil. The association between the civil register and such variables as place of births, sex of child, civil status and residential zone of mother was also studies. It is concluded that underregistration is a public health problem the fundamental contributory factors of which, such as education, making the population aware of the importance of the birth certificate and removing the charge on registration, among other things, could contribute to its decrease with a consequent improvement in Public Health planning with considerable benefit to the population.