Background: The aims of this study were to describe the prevalence, extent and severity of periodontal disease among middle-aged adults, and to examine periodontitis by dental visit pattern, dental and health behaviour, socio-demographics and socioeconomic status. Methods: A random sample of 45-54 year olds from metropolitan Adelaide, South Australia was surveyed by mailed self-complete questionnaire during 2004-2005 with up to four follow-up mailings of the questionnaire to non-respondents (n=879 responded, response rate=43.8 per cent). Oral examinations were performed on 709 people who responded to the questionnaire (completion rate=80.7 per cent), providing an assessment of periodontal status. Results: Prevalence of loss of attachment (LOA) of 6+ mm was 19.2 per cent, extent of sites with LOA of 6+ mm was 1.3 per cent, and severity of LOA of sites with LOA of 2+ mm was 2.4mm. Using a case definition for periodontitis of two or more sites with LOA of 5+ mm and one or more sites with PD of 4+ mm in a multivariate logistic regression showed higher odds of periodontitis for people who last visited for relief of pain (OR=1.93) and who smoked daily/occasionally (OR=3.84), while lower odds were observed for people who were born in Australia (OR=0.51) and spoke English as the main language at home (OR=0.34). Conclusions: While periodontal disease was related to visit pattern and health-related behaviours, the relationship with place of birth and main language spoken at home indicated socio-cultural variation in disease not explained by behaviour among this cohort of 45-54 year olds.Key words: Periodontal disease, 45-54 year olds, visit pattern, smoking status, socio-demographics.Abbreviations and acronyms: CEJ = cemento-enamel junction; CPITN = Community Periodontal Index of Treatment Needs; FGM = free gingival margin; GR = gingival recession; ICC = intra-class correlation; LOA = loss of attachment; PD = pocket depth.