OBJECTIVE: To evaluate the efficacy, tolerability, and pharmacokinetics of bicalutamide plus anastrozole in young males with testotoxicosis. METHODS: This was a multicenter, open-label, single-arm, 12-month, Phase II pilot trial in 14 males (2-9 years) with testotoxicosis treated with bicalutamide (12.5, 25, 50, or 100 mg) and anastrozole (0.5 or 1 mg) daily. The primary outcome was change in growth rate. RESULTS: At 1 year, the mean (standard deviation) change from baseline in growth rate was -1.6 ( 5.1) cm/year and -0.1 ( 1.8) SD units, and in bone maturation was -2.3 ( 0.5) years. The bone age/chronological age ratio was reduced from 2.1 ( 0.6) at baseline to 1.0 ( 0.4) (p = 0.00013). Steady-state trough R-bicalutamide and anastrozole concentrations were attained by Day 21 and 8, respectively. Gynecomastia (42.9%) and breast tenderness (12.5%) were the most common treatment-related adverse events. CONCLUSIONS: Treatment of testotoxicosis with bicalutamide plus anastrozole resulted in slower growth rate.