Various rules have been proposed to identify and exclude live births with implausible values of birth weight for gestational age from large perinatal data sets. The authors carried out a preliminary evaluation of common rules by examining the frequency and nature of rule-based exclusions among live births in Canada (excluding Ontario) between 1992 and 1994. There were 625 (0.09%), 133 (0.02%), 170 (0.02%), and 2,858 (0.40%) live births identified for exclusion by a median birth weight for gestational age +/-4 standard deviations (SD) rule, a +/-5 SD rule, a rule based on expert clinical opinion, and a modification of Tukey's rule, respectively. The birth weight and gestational age distribution of the exclusions depended on the particular rule used; for example, 12.1% and 0.3% of live births of > or =4,500 g were excluded under Tukey's rule and the rule based on expert opinion, respectively. Infant mortality rates among those excluded were 8-13 times higher than among all live births. Current rules for identifying implausible birth weight for gestational age tend to flag live births at high risk for infant death. Such rules may erroneously attenuate temporal trends in important perinatal outcomes.