External apical root resorption (EARR) is a common and occasionally critical problem in orthodontic patients. Mechanical forces compress the periodontium, leading to localized resorption of cementum that exposes dentin to destruction by clastic activity. Factors controlling occurrence and extent of EARR are poorly understood, but there may be a familial (genetic) factor in susceptibility. A sample of full siblings (103 pairs) was studied, all of whom were treated with the same technique by one orthodontist. Crown and root lengths were measured on cephalograms and panoral films before and after treatment. Six roots were scored on each patient, and decrease in root length was the dependent variable. Generalized linear models were used to quantify within and among sibship variances while controlling for sex, age, and severity of malocclusion (FMA, ANB, AOBO, overjet, NAP) as covariates. Results showed significantly greater among-than within-sibship variances, meaning there is a substantive genetic factor in susceptibility to EARR. Heritability estimates were fairly high, averaging 70% for three roots, although low for the mandibular incisor, probably because of little variation. No evidence was found for a sex or age difference in susceptibility. Quantification of a transmissible component suggests it would be useful to search for the biochemical factors controlling the familial differences in susceptibility.