The aim of this study was to evaluate the influence of students' reading literacy, measured by the PIRLS (Progress in International Reading Literacy Study) test, on their performance in the TIMSS (Trends in International Mathematics and Science Study) mathematics and science tests. The data on 4,125 Italian students from 199 schools were analyzed: the sample was representative of students in Grade 4, who participated both in TIMSS and PIRLS 2011. Italian descriptive results were compared with those of other countries that participated in both projects. The results showed a high correlation among all three subjects. Repeated measure variance analyses were conducted to determine the effect of reading literacy on mathematics and science performance on the Italian sample. As far as Italian students were concerned, results confirmed the influence of reading literacy on mathematics achievement, whereas the influence of reading literacy on science achievement was not clear.