This paper aims at analysing the structural relationships among some latent and observed variables related to the assessment of written language performance in 139 fourth grade students of Elementary School selected from nine public schools of the northwest of Mexico. Questionnaires were also applied to the children's parents and teachers. The resulting model confirmed the structure of five latent variables: Parents' educational level, parents' support perceived by the children, parents' support perceived by the parents, parents' support perceived by the teachers, and written language performance. Good convergent construct validity was obtained in each one of these variables. It was also found that the latent variable "parents' support perceived by the children" and "parents' support perceived by the teachers", were significantly explaining the latent variable "performance in written language" (regression coefficients = 0.40 and 0.23). Furthermore, students' performance in written language was explained in a significant way by the teacher' self-evaluation about their knowledge and about teaching according to the Spanish language curriculum in fourth grade (agreement). No significant relationship between parents' educational level and written language performance was found. (Contains 3 figures and 4 tables.)