There is evidence suggesting that missing school negatively relates to academic achievement. However, it is a difficult task to derive unbiased empirical estimates of absences in their influence on performance. One particular challenge arises from the unobserved heterogeneity in the family environment, which may relate to both absence behavior and school performance. This article provides the first analysis aimed at reducing the family-specific omitted variable bias pertaining to measures of absences in their influence on standardized testing achievement. It does so by employing a model of family fixed effects on a longitudinal sample of siblings within the same household in a large urban school district over six years of observations. The results indicate a stronger, statistically significant negative relationship between absences and achievement than what would have been suggested otherwise. Implications are discussed. (Contains 7 notes and 7 tables.)