Earlier studies suggest that children and adults differ in the propensity to develop gingivitis when oral hygiene is abandoned. To confirm the existence of such a difference, a comparative study of pre-school children and young adults was made with objective registration methods. The author performed all registrations. After a period of intensive oral hygiene, all cleaning of teeth was discontinued for 21 days. The amount of bacterial plaque, the amount of gingival exudate and crevicular leukocytes and the bleeding tendency were registered on days 0, 7, 14 and 21. During the experiment the amount of bacterial plaque increased continously in both groups. The amount of gingival exudate and the tendency to gingival bleeding increased to high values in the adults, while only a small rise was seen in the children. The amount of crevicular leukocytes increased in both groups, but the increment was greater in the adults. A comparison concerning differences in gingival exudate and bleeding tendency between pre-school children and adults was undertaken for gingival units that showed a similar plaque development. Under these statistically acceptable prerequisites, it was shown that there is a real difference in the tendency to develop gingivitis between pre-school children and adults.