This article describes a Computerized Adaptive Test (CAT) of Provinha Brasil – Leitura,1 based on Item Response Theory. We detail the operation and development of the algorithm. The CAT was administered by means of tablet computers to 1,983 students in the 1st and 2nd grades of primary education, in 15 schools of the Municipal Education System of São Paulo. Results confirm the quality of Provinha Brasil’s items...