BACKGROUND Tonsillectomy is associated with postoperative nausea and vomiting (PONV) if no prophylaxis is administered. Previous studies have shown that a single dose of dexamethasone decreases the incidence of PONV. The most effective dose of dexamethasone to affect clinical outcome is yet to be defined. METHODS One-hundred-twenty-five children were enrolled in a double-blind, prospective, randomized, dose-escalating study of dexamethasone: 0.0625, 0.125, 0.25, 0.5, or 1 mg/kg, maximum dose 24 mg. Nonparametric ANOVA was used to analyze the incidence of vomiting by treatment group for 0 to < or =5 h, >5 to 24 h. The Cox Proportional Likelihood Ratio Test was used to compare the time of first vomit and time to first pain medication across treatment groups. RESULTS There was no difference in the incidence of vomiting for the five escalating doses of dexamethasone in the time period. There were no differences in secondary outcomes (analgesic requirements, time to first liquid, and change in voice) across treatment groups. CONCLUSION We conclude that the lowest dose of dexamethasone (0.0625 mg/kg) was as effective as the highest dose of dexamethasone (1.0 mg/kg) for preventing PONV or reducing the incidence of other secondary outcomes following tonsillectomy or adenotonsillectomy. There is no justification for the use of high-dose dexamethasone for the prevention of PONV in this cohort of children.