The purpose of this investigation was to study clinically as well as microbiologically the effect of sibling relationship on the periodontal condition in a young population with a relatively high prevalence of periodontal disease and deprived from regular dental care. In this study, 23 family units consisting of 3 more siblings were evaluated. In all, 78 subjects aged 15 to 25 years were included in the study. The mean interproximal amount of loss of attachment in this population was 0.29 mm. The individual mean ranged from 0 to 1.27 mm. In 33% of the subjects, > or = 1 sites with a probing depth of 5 mm or more in conjunction with 2 mm of attachment loss were present. The results show a significant sibship effect for: plaque, calculus, loss of attachment, spirochetes on the tongue and in the pocket, Porphyromonas gingivalis on the gingiva and in the saliva and Prevotella intermedia in the saliva. These results support the hypothesis that periodontitis aggregates in families.