The study examines the effect of moderate to high worm burdens of Trichuris trichiura infection on the cognitive functions of 159 school children (age 9-12 years) in Jamaica, using a double-blind placebo-controlled protocol. Results were evaluated by using a forward-stepwise multiple linear regression. Removal of worms led to a significant improvement in tests of auditory short-term memory (p less than 0.017; p less than 0.013), and scanning and retrieval of long-term memory (p less than 0.001). Nine weeks after treatment, there were no longer significant differences between the treated children and an uninfected Control group in these three tests of cognitive function. It is concluded that whipworm infection has an adverse effect on certain cognitive functions which is reversible by therapy.