Dexamethasone has been widely used before general anesthesia induction. However, previous studies have found that a pre-induction bolus dose of dexamethasone sometimes causes perineal pruritus. We hypothesized that an appropriate prolongation of the injection time might suppress dexamethasone-induced perineal pruritus. Four hundred patients requiring general anaesthesia were randomly allocated into four groups: group I receiving 2 ml dexamethasone (5 mg/ml); group II receiving 4 ml dexamethasone (2.5mg/ml); group III receiving 10 ml dexamethasone (1 mg/ml); and group IV receiving 20 ml dexamethasone (0.5 mg/ml). Dexamethasone was diluted with 0.9% sodium chloride. The injection time of dexamethasone was 5s in groups I, II and III; while the injection time of dexamethasone was 30s in group IV. Occurrence of perineal pruritus was significantly reduced in Group IV (0% vs 38%, 32% and 12% in Groups I, II and III, respectively, p < 0.05). The incidence was higher in females than in males (p < 0.05). The duration of perineal pruritus was longer in females than in males in groups I, II and III (p < 0.05). We conclude that the dilution of dexamethasone to 0.5 mg/ml with 0.9% sodium chloride combined with prolonged injection time to 30 s eliminates dexamethasone-induced perineal pruritus.