A clinical trial was undertaken to assess whether crowded teeth were more likely to accumulate plaque and develop gingivitis than non-crowded teeth. A tooth was considered crowded if it was displaced by 2 mm and/or rotated 15 degrees or more from the normal position in the arch. The material consisted of 50 dental students aged 21 to 32 years. An initial examination comprised assessment of Plaque Index, Gingival Index and pocket depths. After this examination the subjects refrained from using interdental cleaning aids but continued with their normal toothbrushing for 40 days. After re-examination they were instructed in the effective use of dental floss. A final examination was carried out after 140 days. At the start of the trial no difference was found in regard to the Plaque Index between crowded and non-crowded teeth. The Gingival Index for growded front teeth, but not for crowded premolars, was somewhat higher than for the corresponding controls. The cessation of interdental cleaning resulted in a similar increase in plaque accumulation and gingival inflammation in both non-crowded and crowded teeth. The use of dental floss for approximal tooth cleaning resulted in a similar decrease in the Plaque- and Gingival Indices for both types of teeth. The results demonstrate that in a group of young adults, crowding of teeth (1) did not favour plaque accumulation on approximal tooth surfaces and (2) influenced the degree of gingival inflammation only to a minor extent.