To determine the degree of intraspecimen fecal egg count variation in Schistosoma mansoni infection and its impact on commonly used parasitologic parameters obtained by single egg counts, 10 25-mg Kato-Katz slides were prepared from each of three stool specimens collected on different days in a study group of 20 infected people. Individual fecal egg counts in these series of examinations varied considerably and this had profound consequences for the reliability of both qualitative and quantitative diagnosis. In light infections, S. mansoni eggs in stools appeared to be homogeneously mixed. However, this distribution became heterogeneous as the intensity of infection increased, indicating clustering of eggs in stool. The cumulative egg counts in the 10 slides of the same 20 people examined in this study were compared with those in 14 slides prepared from seven stool samples collected on different days. This revealed significantly different mean egg counts for six people, even after such exhaustive series of examinations. Intraspecimen variation also biased considerably some operational parameters used to determine the infection status at the group level, particularly when these were determined by the examination of single 25-mg slides. The examination of duplicate or multiple slides improved the intraspecimen estimates of these parameters but did not overcome day-to-day variation. The examination of fewer samples taken on different days proved to be more adequate than examining more slides from one stool specimen for the determination of precise estimates of the real infection status.