Different periodontitis definitions have been used in periodontal research. This study assessed the impact of case definition on the prevalence and extent rates of periodontitis. A data set including 340 periodontal records, collected in Belo Horizonte, Brazil, was used. Periodontitis was defined as: 1) one site with probing depth (PD) ≥ 4 mm; 2) clinical attachment level (CAL) ≥ 5 mm in ≥ 4 sites + one site with PD ≥ 4 mm; 3) CAL ≥ 6 mm in ≥ 2 teeth + one site with PD ≥ 5 mm; 4) ≥ 4 teeth with ≥ 1 sites with PD ≥ 4 mm + CAL ≥ 3 mm; 5a) interproximal CAL or PD ≥ 4 mm at ≥ 2 sites, not on the same tooth; and 5b) interproximal CAL of ≥ 6 mm at ≥ 2 sites, not on the same tooth + PD ≥ 5 mm at ≥ 1 proximal site. Definition 5 was determined to be the gold standard and the definitions were compared by means of agreement, sensitivity, specificity, and positive and negative predictive values. Prevalence and extent rates greatly varied, from 13.8% to 65.3% and from 9.7% to 55.6%, respectively. The use of different case definitions has a great impact on the prevalence and extent rates of periodontitis. (J Oral Sci 51, 199-206, 2009)