A majority of previous toothbrushing studies have examined proposed virtues of particular brushing techniques or the relative cleaning ability of various toothbrushes. A smaller number were concerned with their role in prevention of gingivitis. Whether toothbrushing could be therapeutic in naturally occurring gingivitis was tested in 40 teenagers aged 12 to 14 years. Groups from two public schools were examined for gingivitis and plaque. Students from both groups were given a soft multi-tufted toothbrush and nonfluoridated toothpaste to be used daily. Students at one school were given a single intensive oral hygiene session. All students were reexamined 42 days later and given a thorough prophylaxis. Final readings were obtained 84 days after baseline and the results analyzed statistically. Sign test analysis disclosed a significant (P less than .01) improvement in plaque and gingivitis scores for both groups. Wilcoxon tests of patient means revealed a significant improvement in plaque scores, whereas gingivitis means were more variable. Chi square analysis between groups showed that the groups receiving oral hygiene instructions had significantly better (P less than .01) plaque and gingivitis scores at session 2. Gingivitis differences were generally significant at session 3. Toothbrushing has a therapeutic effect on naturally occurring gingivitis, enhanced by oral hygiene instruction.