The course of Graves' thyrotoxicosis in 7 prepubertal children (6.4+/-2.4 yr) was compared with that in 21 pubertal (12.5+/-1.1 yr) and 12 postpubertal (16.2+/-0.84 yr) patients. In the prepubertal group the main complaints were weight loss and frequent bowel movements (86%), whereas typical symptoms (irritability, palpitations, heat intolerance, and neck lump) occurred significantly less often (P < 0.01). The most prominent manifestation at diagnosis was accelerated growth and bone maturation: their height SD score was significantly greater than that of the pubertal and postpubertal patients (2.6+/-0.7 us. 0.15+/-0.65 and 0.15+/-0.9, respectively, P < 0.001), and their bone age to chronological age ratio was 1.39+/-0.35 compared with 0.98+/-0.06 in the pubertal children (P = 0.02). T3 levels were also significantly higher than in the other two groups (9.9+/-2.9 nmol/L vs. 6.32+/-1.9 nmol/L and 6.02+/-2.0 nmol/L, P = 0.01). All patients were initially prescribed antithyroid drugs (ATDs). Overall, adverse reactions to ATDs occurred in 35%, with a higher rate among the prepubertal children (71%) than the pubertal (28%) and postpubertal (25%) patients (P = 0.08). Major adverse reactions were noted in two children, both prepubertal. Remission was achieved in 10 patients (28%). Although the rate of remission did not differ among the three groups, time to remission tended to be longer in the prepubertal children (P = 0.09). In conclusion, thyrotoxicosis has an atypical presentation and more severe course in prepubertal children. Considering their adverse reactions to ATD, overall low remission rate, and long period to remission, definitive treatment should be considered earlier in this age group.