We evaluated the association between radiographically assessed extension and density of root canal fillings and postoperative apical radiolucencies (ARs) by using data from 288 participants in the Veterans Affairs Dental Longitudinal Study. Study subjects were not Veterans Affairs patients; all received their medical and dental care in the private sector. Generalized estimating equations were used to account for multiple teeth within subjects and to control for covariates of interest. Defective root filling density was associated with increased odds of postoperative AR among teeth with no preoperative AR (odds ratio, 3.0; 95% confidence interval [CI], 1.3-7.1), although preoperative AR was the strongest risk factor for postoperative AR (odds ratio, 29.2; 95% CI, 13.6-63.0 among teeth with ideal density). Compared with well-extended root fillings, neither overextended nor underextended root fillings separately were related to postoperative AR, but when those 2 categories were collapsed into one poorly extended category, poor extension was related to postoperative AR (odds ratio, 1.8; 95% CI, 1.1-3.2).