This study presents the clinical findings and the distribution of prepubertal periodontitis in an extended family with high prevalence of this entity. The expression of surface markers and adhesion molecules on peripheral lymphocytes were also studied. Approximately 50% of the children in this family suffered from prepubertal periodontitis. All the affected children were otherwise healthy. 2 identical twins were similarly, but not identically, affected. Detailed laboratory tests and analysis of lymphocyte surface marker expression, including CD18, were all within the normal levels. Both localized and generalized forms of prepubertal periodontitis were found. The high prevalence of prepubertal periodontitis in the 2 branches of this family, and the fact that identical twins were similarly affected, suggest a strong genetic predisposition for prepubertal periodontitis. The family pedigree is consistent with an autosomal-dominant mode of transmission. The coexistence of localized and generalized forms of the disease in sibs suggests the same genetic etiology for both entities with variability in disease expression. This variability in disease expression is further supported by the fact that 2 identical twins were not identically affected.