An assessment of the oral hygiene and periodontal health was made in 1596 school children (777 boys and 819 girls) aged 7-14 years from randomly selected classes and schools in Plovdiv, Bulgaria. The oral hygiene status of the school children assessed by the plaque indexes (PLI) of J. Greene and J. Vermillion, and that of J. Silness and H. Löe was found to be rather poor. The relative share of subjects with plaque index of 0.00 to 1.00 was found to increase with age for both sexes. The relative share of children with chronic catarrhal gingivitis was 42.61 +/- 2.34 indicating the necessity of active prophylaxis and treatment of the disease. A tendency was observed towards an increase of the disease incidence in comparison with data obtained in a 1983 study. A correlation between the hygiene status and chronic gingival inflammation in 14-year-old school children was found.