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The prevalence of craniomandibular disorders was studied in 285 17-year-old adolescents with the aid of a questionnaire and a functional examination of the masticatory system including evaluation of TMJs, masticatory muscles, mandibular mobility, and occlusion. The adolescents were followed up longitudinally two more subsequent years. Totally 27 subjects dropped out, leaving 258 for the longitudinal intraindividual comparisons. At the age of 17 a fifth of the subjects reported some symptoms involving the masticatory system, of which most were mild, according to the anamnestic index (Ai) used. Oral parafunctions were commonly reported of which nail-biting dominated. Grinding and clenching of teeth were reported by 8% and 11% respectively. Signs of mandibular dysfunction were found in 56% of the adolescents and were mostly mild according to the dysfunction index (Di) used. Girls more often had signs of mandibular dysfunction than boys. Morphologic malocclusion was recorded in 35%, unilateral contact in RP in 77%, lateral shift between RP and IP greater than or equal to 0.5 mm in 19% and mediotrusion interferences in 30% among the 17-year-olds. No significant difference between sexes was found. Of the sample 62% had either some sign or symptom of dysfunction and there was a positive relationship between the dysfunction indices used. Neither morphologic nor functional malocclusions were related to the Ai. TMJ sounds were related to palpation tenderness in the lateral pterygoid muscle and impaired mobility of the mandible. The number of masticatory muscles tender to palpation was related to reports of fatigue in the jaw, TMJ tenderness, and mediotrusion interferences. Recurrent headache was reported by about 18% of the girls and by almost 6% of the boys. Fatigue in the jaws and difficulties in chewing were commoner in those with frequent and more intensive headache. Tenderness to palpation of the masticatory muscles and impaired mandibular mobility were significantly commoner findings among those with recurrent headaches and those with more intense headache. Tooth-grinding and clenching were related to frequency but not to intensity of headache. Reports of TMJ sounds increased with age for girls who also more frequently reported recurrent headaches than boys. The prevalence of symptoms of mandibular dysfunction was about 20% each year, but there was no general increase of frequency and severity of symptoms during the observation period in spite of an incidence of 8%. The prevalence was, according to the Ai, significantly higher for 18- and 19-year-old girls than for boys. Most symptoms were mild and fluctuated longitudinally.(ABSTRACT TRUNCATED AT 400 WORDS)