The effect of mass treatment on questionnaire results in the diagnosis of schistosomiasis mansoni was examined in 267 school children in an endemic area of Tanzania by Kato-Katz analysis of fecal specimens. The questionnaire asked for information about self-diagnosis, abdominal symptoms, blood in stools, history of wild water contact, stool examination and medication for schistosomiasis, and knowledge of the disease. A logistic regression analysis disclosed a significant association between schistosomiasis and "diarrhea" (p ≈ 0.007; odds ratio, 32.0; confidence interval, 2.5 - 403.3) and "abdominal enlargement" (p ≈ 0.003; odds ratio, 15.2; confidence interval, 2.6 - 90.1) among 61 children who had no history of medication for schistosomiasis. The sensitivity and specificity of the model were 86% and 64%, respectively. In contrast, no significant correlation was observed either for the 116 treated children, or for all the 267 children after the mass treatment. We conclude, therefore, that for children who had no history of medication for schistosomiasis, the questionnaire for abdominal manifestations provides reliable information on S. mansoni infection. However, once a child takes medication, the questionnaire becomes unreliable. This observation suggests that immunomodulation by anti-schistosomiasis drugs that kill adult worms exerts an effect on the appearance of abdominal manifestations and might explain the ambiguity of clinical symptoms in chronically infested patients, except in terminal cases. Further studies are required to develop a simple, rapid and cost-effective diagnostic method for monitoring S. mansoni infection after medication in local areas without resort to laboratory-based identification of schistosomiasis.