The aim of the present study was to examine whether school performance is influenced by absence because of illness and if there are any differences in the relationship due to grade, sex, and season of birth. Data were taken for 598 pupils born in 1947 whose complete records for the 1st through 6th grades were stored at the city archive of Uppsala. Simple correlational analyses were conducted between spring absence due to illness in the 2nd through 6th grades and teacher-assigned grades during the same spring for verbal and arithmetic performance. Significant negative correlations were found between absence and both arithmetic and verbal performance in Grades 5 and 6. Subgroup analysis revealed significant correlations for the girls in the 5th grade and for boys in the 6th grade. However, only girls and boys born during the first half of the year showed these significant correlations between absence due to illness and performance.