The aim of this study was to determine whether dental caries and periodontal disease occur frequently in the same subjects 1469 young people, aged 7, 12 and 15-19 years, and representing the urban (60%) and rural (40%) population from Spain were evaluated. Dental caries and periodontal treatment needs were registered according to the index D.M.F.T. and C.P.I.T.N., following the criteria of WHO. We have found a highly significant relation between both variables, average D.M.F.T. scores increased as periodontal treatment needs were more advanced. Those subjects with lower codes of C.P.I.T.N. presented also lower D.M.F.T. scores, while these individuals with higher D.M.F.T. scores, based on a great number of open carious lesions, were more affected for periodontal disease. It does not support either a directly synergistic or antagonistic correlation in the prevalence of these two diseases in the same subjects. Dental health status is at a certain time an end result of years of varying dental health behaviour. In industrialized countries a variety of factors seems to determine the prevalence of dental caries and/or periodontal disease: dietary habits, access to fluorides, efficacy of oral hygiene measures, utilization of dental health care services. These data and ideas supported the strategy of preventing the dental caries and periodontal disease simultaneously.