This study examined the effect of folic acid supplementation on the recurrence of phenytoin-induced gingival overgrowth following gingivectomy. 8 residents of an institution for the developmentally disabled were randomly assigned to a treatment (N = 4) or control (N = 4) group. Subjects in the treatment group received an oral supplementation of 5 mg of folic acid daily during the study; those in the control group did not. A gingivectomy with an external beveled incision made to the crest of the alveolus was completed by quadrants. The following data were obtained prior to gingivectomy, 2 weeks following the last quadrant of surgery, and at 3 and 6 months post-surgery: plaque and gingival index scores, red blood cell folic acid levels, free phenytoin blood levels, photographs, and impressions. % change in overgrowth was determined from cross-sectional area measurements made on dies obtained from bucco-lingual cuts on stone models. Differences across time between and within groups were tested by a two-factor repeated measure analysis of variance. The groups did not differ in plaque and gingival index scores or free phenytoin blood levels. The treatment group had significantly higher red blood cell folic acid levels (p less than or equal to 0.0001). Reduction in gingival overgrowth as a result of surgery was similar in both groups. Although the treatment group had significantly less recurrence of gingival overgrowth (p less than or equal to 0.05), the mean differences amounted to only 6-7% at 3 and 6 months.