The aim of this study was to observe the relationship between the anesthetic block sucess (ABS) of mandibular nerve, by injecting 1.8 ml of lidocaine 2% with epinephrine 1:100,000, in children and their body mass index (BMI). A prospective cohort study was designed. A sample of 93 6-13 years children attended in JUNAEB Dental Module of Frutillar between April and July 2011. We recorded age, gender, reason for tooth extraction and tooth type (permanent or deciduous). BMI was calculated for each child and categorized into two cohorts of study according to WHO criteria: those with a BMI <p85 were defined as "low - normal weight" (LNW) and those with a BMI equal or &gt;p85 as "over weight - obese" (OWO). After applying the anesthetic technique, effect was evaluated at 10 minutes recording its effectivenes. We analyzed statistically significant relationship between ABS and BMI using Fisher's exact test with (p <0.05). Anesthetic effect was achieved in 84 children (90.3%) with a 89.36% in LNW group and 91.30% in OWO group. There was no statistically significant relationship between ABS and BMI (p=1.0). There is no relationship between anesthetic effectiveness of mandibular nerve and body mass index in children.