The objective of this study was to assess the relative effects of age and oral hygiene on the progression of periodontitis by estimating incidence from age-specific prevalence. This study analyzed data from a representative national sample of 14,690 dentate Americans, aged from 15 to 74, seen in the first National Health and Nutrition Examination Survey (NHANES I) in 1971-1974. Results showed that the rate of increase in the estimated incidence of periodontitis with age, throughout all age groups, is much higher among subjects with poor oral hygiene than among those with good oral hygiene. Oral hygiene was confirmed as the most important predictor for periodontitis; in all age groups, more than 95% of those examined with good oral hygiene did not have periodontitis. It was concluded that the effect of age on the progression of periodontitis could therefore be considered negligible when good oral hygiene is maintained.