This paper analyses the associations between computer use in schools and at home and test scores by using TIMSS data covering over 900,000 children in fourth grade. When controlling for school fixed effects, pupils who use computers at school, especially those who use them frequently are found to achieve less than students who never use computers. Daily computer use at home is negatively associated with test scores, although monthly, and sometimes weekly, use is positively associated with pupil performance. There is no significant difference between subjects and only small gender and country differences are observed. Moreover, the result suggests that the negative association of computer use at school is larger among low-performing pupils than for high-performing pupils. The findings suggest a negative association of computer use at school and test scores but do not reject the possibility that computers have a positive impact on test scores if computers are used optimally.