We examined the type of errors on multiplication and division computation problems of 326 rising fifth graders enrolled in four elementary schools in Northern Portugal. We further examined whether there was a difference in the number of errors across age and whether there was an association between students' performance on number knowledge and multiplication and division computation problems. Error analysis of students' responses indicated that miscalculation and no attempt to solve the problem were the two most frequent error types. We found that older students made more errors compared to younger students. We argue that knowledge of individual student error types is critical to making sound instructional decisions. Based on the results of the present study, we discuss implications for future research and classroom practice.