Infection and re-infection patterns were evaluated in a recent Schistosoma mansoni focus in northern Senegal, by determining concentrations of serum circulating anodic antigen (CAA), as a measure of worm burden, and counting eggs in faeces before, 6 or 12 weeks and 1 year after praziquantel treatment in two subsequent cohorts (cohort A and B). No differences in egg counts and CAA concentrations or their relationship were found between the cohorts, which were examined 2 years apart. Within both cohorts, CAA concentrations showed the same, typical, age-related patterns as egg counts, with a peak in children and a strong decline in adults. These trends were apparent both before and 1 year after treatment. The results indicate that an age-related resistance to infection and to re-infection has been firmly established, at a steady level, in the recent S. mansoni focus investigated, with no indication of a gradual development of immunity or anti-fecundity immunity over a period of 2 years. Both shortly and 1 year after treatment, the decrease in egg counts was stronger than that in CAA concentrations, indicating that that there had been a reduction in worm fecundity after treatment. The possibility that praziquantel may induce anti-fecundity immunity has important implications for the use and interpretation of the results of (egg-count-based) re-infection studies designed to follow the development of naturally acquired immunity.