To analyze the occurrence of root caries in relation to the periodontal state, we examined clinically a representative sample (n = 4,777) of Finish adults, aged 30 years and older. Our definition of root caries included primary decay on root surfaces, and a subject's periodontal state was described according to the presence or absence of gingival inflammation or periodontal pockets and subgingival calculus and/or overhangings. Subjects with a healthy periodontium seldom (4%) had any caries on root surfaces, whereas 15% had caries in conjunction with gingival inflammation, and 17% in conjunction with deepened pockets. The deeper the pockets, the greater the frequency of root caries for men, but not for women. Subgingival plaque retention showed a strong association with root caries. As the study population represented Finnish adults, the results can be generalized with reasonable confidence.