A 4-year follow-up study of academic achievement in children aged between 11 and 17 years with epilepsy or asthma was carried out to identify differences between the two samples and to identify change in achievement over time. Differences based on sex and seizure severity also were explored. There were 98 subjects in the group with epilepsy and 96 subjects in the group with asthma. Academic achievement in five areas (Composite, Reading, Mathematics, Language, and Vocabulary) was measured using school-administered group test scores. To explore change over time in condition severity, each child was categorized as having a low or high condition severity at baseline (time I) and again 4 years later, resulting in four groups: low–low, low–high, high–low, and high–high. There were too few cases in the low–high group to be included in the analyses. Data were processed using analysis of covariance (ANCOVA), intraclass correlation coefficients, and paired t tests. At follow-up the children with epilepsy continued to perform significantly worse in all five achievement areas than the children with asthma. Children with either inactive or low-severity epilepsy had mean scores comparable to national norms; those with high seizure severity had mean scores ranging from 3 to 5 points below national norms. No changes were found in academic achievement over time for either sample, even among those whose conditions improved. Although boys with high-severity epilepsy continued to have the lowest achievement scores, there was no trend for them to decline in achievement over time.