The aim of this study was to find the optimal approach to the use of the Community Periodontal Index of Treatment Needs (CPITN) in age groups under 20 years of age in which the presence of false pockets around erupting teeth has caused confusion. Fifty boys and fifty girls in each age group of 7-, 12- and 17 years were selected at random from among young patients attending for regular dental care. To allow detailed analysis, CPITN recordings were made for each tooth instead of per sextant. The 4-5 mm pockets were divided into true pathologic pockets requiring scaling and improved oral hygiene; false pockets with bleeding after gentle probing and thus needing improved oral hygiene only; and false pockets with no treatment need. Comparisons of periodontal treatment needs per sextant were made after basing the CPITN code number on examination of all teeth in the sextant, of the CPITN epidemiological index teeth (17, 16; 11; 26, 27; 37, 36; 31; 46, 47), and of teeth 16, 11, 26, 36, 31, 46. Incomplete eruption even of first molars and maxillary central incisors was frequently observed at age 7 whereas at ages 12 and 17 the presence of false pockets was most frequent around mandibular and maxillary second molars. It is suggested that in populations under 20 years of age CPITN recordings should be based on examination of teeth 16, 11, 26, 36, 31, 46.(ABSTRACT TRUNCATED AT 250 WORDS)