OBJECTIVES This study assessed the relationship between incomplete birth certificates and infant mortality. METHODS Birth certificates from California (n = 538 945) were assessed in regard to underreporting of 13 predictors of perinatal outcomes and mortality. RESULTS Of the birth certificates studied, 7.25% were incomplete. Underreporting was most common in the case of women at high risk for poor perinatal outcomes and infants dying within the first day. Increasing numbers of unreported items were shown to be associated with corresponding increases in neonatal and postneonatal mortality rates. CONCLUSIONS Incomplete birth certificates provide an important marker for identifying high-risk women and vulnerable infants. Because data "cleaning" will result in the removal of mothers and infants at highest risk, birth certificate analyses should include incomplete records.