OBJECTIVE To investigate oral hygiene and periodontal status of 12- and 15-year old Greek adolescents, in relation to sociodemographic and behavioural parameters. METHODS A stratified cluster sample of 1,224 12-year old and 1,257 15-year old adolescents of Greek nationality were selected and examined by calibrated examiners. Periodontal and oral hygiene status were assessed using the Community Periodontal Index (CPI) and the simplified Debris Index (DIs) respectively. The socio-demographic and behavioural data collected included region, location, gender, parental educational level, tooth brushing frequency and reason for dental attendance. RESULTS The majority of adolescents aged 12 (75.0%) and 15-years (61.4%) had fair oral hygiene levels. The most frequently observed condition in both ages was calculus with or without bleeding (42.8% in the younger and 53.3% in the older age group). Bleeding on probing was found in 41.5% of the 12-year-olds and in 30.0% of the 15-year-olds. The occurrence of shallow and/ or deep periodontal pockets was very low (0.2%). Multivariable modelling revealed that gender, location and tooth brushing frequency were strongly associated with oral hygiene status in both ages; girls, those living in urban areas and brushing teeth more frequently had significantly lower DI-s. Tooth brushing frequency was also associated with periodontal status in both ages, while living in urban areas was associated with better periodontal health only in the 15-year-olds. CONCLUSIONS The study demonstrated that oral hygiene conditions among Greek children and adolescents are not satisfactory and that the occurrence of gingivitis is high. More efforts on oral health education and oral hygiene instruction are needed to improve their periodontal and oral hygiene status.