To determine the prevalence and risk factors for (i) overweight/obesity and (ii) weight gain six months after transplantation and to study the effect of weight excess on graft function and survival. We performed a retrospective study of kidney transplanted children. Endpoints: (i) prevalence of overweight/obesity at sixth month, (ii) gaining 1.0 BMI SDS from one to six months. To study the effects of weight excess, graft function and survival at 36 months were the endpoints. The study included 197 individuals. At sixth month, 57/197 (29%) presented overweight/obesity, and the factors associated to this outcome were: (i) age at transplantation (OR = 3.04) and (ii) overweight/obesity in the first month (OR = 22.16). Groups presented no difference on graft function and survival at 36 months. From one to six months, 90/197 (46%) patients gained >1.0 BMI SDS. This outcome was associated with (i) female sex (OR = 2.50), (ii) steroids' pulses (OR = 2.98), (iii) steroids exposure (OR = 1.04), and (iv) living donor (OR = 2.69). The group that gained BMI presented a lower 36 months graft survival (86% vs. 98%, p < 0.001). Weight excess and gain after transplantation are frequent, particularly in younger female recipients and in those receiving high steroids exposure. The lower graft survival in patients with rapid weight gain deserves investigation.