Obesity has been linked to several physiological and psycho-social diseases, decreases in cognitive function, poor levels of scholastic achievement, low socioeconomic status and delayed onset of maturity. This study investigates the association between obesity and both academic performance and reasoning ability in 394 male and 398 females students between the ages of 6 and 12 from Lisbon, Portugal. It also assesses how this relationship may be influenced by chronological age, maturity and socioeconomic status. The results suggest that: 1) reasoning ability is independent of socioeconomic status and level of maturity; 2) no differences in reasoning ability exist between groups of different BMI; 3) academic performance is moderated by chronological age in boys, and by maturity in both genders; 4) obesity is not associated with academic performance. It is concluded that reasoning ability and academic performance are not associated with obesity, and that inter-individual differences in academic performance in boys may be explained by differences in their level of maturity.