In a study group of 183 people in a Schistosoma mansoni-endemic area in Burundi, stool examinations were performed with duplicate 25-mg Kato-Katz slides on seven occasions (days 1, 3, 5, 8, 10, 32, and 37). Point prevalences detected by single examinations of 25 mg and 50 mg of stool varied from 41.0% to 57.9% and from 55.7% to 63.9%, respectively. The cumulative prevalence for all seven measurements was 82.0%. The individual day-to-day variation in egg output was important. The majority of infections missed by the examination of single slides and specimens were light ones. The Kato-Katz method applied on a single stool specimen is more suitable for morbidity control, but less suitable for control of infection. When a precise quantitative diagnosis on the individual level is required, several measurements on different days are necessary. The data presented validate recently developed statistical models and charts predicting true prevalences.