The effect of targeted mass treatment, a new strategy for cost-effective control of schistosomiasis mansoni based on administering single-dose chemotherapeutic agents to individuals with disease manifestations (hepatosplenomegaly) or heavy infections, was evaluated in an endemic area in Kenya. Two years after treatment of subjects with hepatosplenomegaly, the mean liver midsternal-line measurement decreased from 6 x 5 +/- 0 x 6 to 2 x 9 +/- 0 x 5 cm and the mean faecal egg count dropped significantly from its pretreatment level of 1090 +/- 290/g to 88 +/- 31/g. Targeted chemotherapy was then administered to a group of 122 subjects with faecal egg counts greater than or equal to 400/g. A similar maintained decrease in egg counts after chemotherapy was demonstrated in this group; mean egg count after one year was 115 +/- 17/g compared with 1250 +/- 232/g before treatment. The yearly rate of acquisition of heavy infection in this community was low (7%) and did not differ significantly in the uninfected, lightly infected, or heavily infected (and treated) groups.