AIM: The purpose of this investigation was to examine the long-term relationship between dental restorations and periodontal health. MATERIAL AND METHODS: The data derived from a 26-year longitudinal study of a group of Scandinavian middle-class males characterized by good to moderate oral hygiene and regular dental check-ups. At each of 7 examinations between 1969 and 1995, the mesial and buccal surfaces were scored for dental, restorative and periodontal parameters. The mesial sites of premolars and molars of 160 participants were observed during 26 years (1969-1995). A control group with 615 sound surfaces or filling margins located more than 1 mm from the gingival margin in all 7 surveys was compared with a test cohort with 98 surfaces which were sound or had filling margins located more than 1 mm from the gingival margin at baseline (1969) and had a subgingival filling margin 2 years after (1971). RESULTS AND CONCLUSIONS: The study confirmed the long held concept that restorations placed below the gingival margin are detrimental to gingival and periodontal health. In addition, this study suggests that the increased loss of attachment found in teeth with subgingival restorations started slowly and could be detected clinically 1 to 3 years after the fabrication and placement of the restorations. A subsequent "burn-out" effect was suggested.