Severe infections by the geohelminth Trichuris trichiura detrimentally affect young children's growth and development. There is concern that mild to moderate infections may affect older children's school performance and nutritional status. We therefore examined the relationship between varying intensities of infection and school achievement, attendance and nutritional status in 616 schoolchildren. A total of 409 children with Trichuris infection of intensities greater than 1200 eggs per gram of stool (epg) were first identified, then for every 2 infected children in a class, an uninfected child was selected. After controlling for socioeconomic status, gender, age, school and the presence of Ascaris infections, the uninfected children had higher reading and arithmetic scores than children with infections of more than 4000 epg and were taller than those with intensities greater than 2000 epg. However, there were no significant differences in spelling, school attendance and body mass index. Although a treatment trial is needed to determine causation, these results indicate that moderate levels of infection are associated with poor school achievement and growth.