Dental plaque growth was observed among 180 teenage boys during a 28-day period following prophylaxis. During this period, subjects brushed their teeth under supervision for 2 minutes daily. Plaque levels were measured immediately after brushing and 24 hours after brushing. Both levels increased rapidly during the first 14 days and appeared to be leveled off at 28 days. Less than half of the plaque was removed with one brushing per day leaving about 60% after brushing to promote rapid regrowth. Regrowth rate after brushing on the 28th day was 0.032 plaque units per hour over a 24-hour period. The regrowth rate for the group brushing with dentifrice was 27% lower than for the group brushing without a dentifrice.