Abstract A sizable body of literature has concluded that males do better in math compared with females. Although differences have narrowed over time, generally speaking males do better on standardized test scores. However, there is no agreement on when such disparity appears and how big the differences are. This paper explores at what point during elementary school gender differences appear, when these become significant and how the gap evolves as children progress at early years. To explain differences by gender, math scores are used. Data comes from the Early Childhood Longitudinal Program (ECLS-K) in the USA. Longitudinal and cross-sectional analysis are conducted. Gender gaps are decomposed through the use of ñopo-match. Results show that gender gaps are almost inexistent at the beginning of schooling but they broaden rapidly. Between first and fifth grade, gender gaps increase by 60.8%. The unexplained components of gender differences increase over time, which suggests that the importance of socioeconomic and school factors decreases as children progress in the school.