A longitudinal study was conducted to evaluate the impact of praziquantel (PZQ) for the treatment of Schistosoma mansoni infection among schoolchildren in Al Gunaid in Central Sudan. A cohort of schoolchildren (6-15 years of age) was investigated before and 1 year after treatment with a single dose of PZQ 40 mg/kg. Parasitological examinations for S. mansoni were performed before and after treatment, and prevalence and intensity of infection were analysed. Of 2741 schoolchildren recruited from six elementary schools at baseline, 2521 were successfully traced and re-examined at follow-up, with two complete sets of longitudinal parasitological data on S. mansoni. Boys showed significantly higher prevalence of S. mansoni infection than girls. A single dose of PZQ reduced the overall prevalence of S. mansoni infection by 36.7% (from 59.1 to 37.4%) and the intensity of infection by 41.1% (from 116.7 to 68.7 eggs per gram of stool) 1 year after treatment. The reduction in prevalence was significantly higher among the group of children with heavy infections (by 76.1%, from 6.7 to 1.6%) and among girls (by 54.1%, 42.3 to 19.4%) at 1 year after treatment. Thus, in spite of a significant reduction in the prevalence and intensity of S. mansoni infection 1 year after PZQ treatment, the prevalence of the disease was still high and further research is needed on this topic.