<p>The authors evaluated the level of cooperation by 255 healthy children (147 boys, 108 girls ;2­9 years old) during their initial examination as well as in 3 subsequent examinations performed at the outpatient clinic of the Pediatric Dentistry Department, Tsurumi University Dental Hospital. Changes in level of cooperation with the progress of treatment were evaluated using Frankl's Behavior Rating Scale, with the following results obtained.</p><p></p><p>1.Overall, Class 3 cooperation had the highest and Class 1 the lowest frequency at each recording point of all 4 visits for each age group. Moreover, the frequency of Class 1 and 2 tended to de­ crease with increasing age.</p><p></p><p>2.When the level of cooperation was classified into 2 groups, namely, "uncooperative" (Class 1 and 2)and "cooperative" (Class 3 and 4), the time point of assessment was an independent factor in 3­ -year-­old children. In addition, the frequency of "cooperative" and "uncooperative" became reversed at the age of 3 years.</p><p></p><p>3.When changes in the level of cooperation in each age group were evaluated, the most frequently observed patterns were "continually cooperative" in all subjects, "continually uncooperative" in 2­ and 3­-year-­old children, and "continually cooperative" in 4­ to 9­-year-­old children. In addition, the frequency of being "continually cooperative" was significantly low in 2­ and 3­-year-­old children and significantly high in 6­ to 9­-year-­old children, that of "becoming cooperative" was signifi­cantly low in 2­-year-­old children and significantly high in 3­-year-­old children, that of "inconsis­tent" was significantly high in 4­ and 5­-year-­old children, and that of "less cooperative" was sig­nificantly high in 4­-year-­old children.</p><p></p><p>Our results indicated that the level of cooperation was low in 2­-year-­old children at every assess­ment point from the first to fourth visits. In 3­-year-­old children, the level of cooperation was low at the first visit but increased as treatments continued. In children 4 years old and older, the level of co­ operation was high from the first visit. Finally, the level of cooperation was nearly satisfactory in all by the time the children reached school age.</p>