Surveys for lymphatic filariasis were carried out for the first time in Lower Shire (Nsanje and Chikawawa Districts) of southern Malawi, in April-June 2000. There were 3 phases. In phase I, questionnaire surveys in 48 randomly selected villages indicated that chronic manifestations of lymphatic filariasis ('swollen scrotum' and 'swollen legs') were common and widespread in the area. In phase II, volunteers from 10 of the villages reporting frequent manifestations of filariasis in phase I were examined with the ICT whole-blood test for Wuchereria bancrofti-specific circulating filarial antigen (CFA). The observed prevalence of CFA positivity was very high (range, 38.8-79.1% for the villages). In phase III, a more detailed parasitological, CFA and clinical investigation was carried out in 2 of the high CFA prevalence villages identified in phase II (1 in each district). Overall, 18.1% and 22.2% were positive for microfilariae, and 62.3% and 64.6% were positive for CFA in the 2 villages. Among those aged > or = 15 years, 3.7% and 1.3% had leg elephantiasis, and 17.9% and 13.0% (of males only) had hydrocoele. In both phase II and III, CFA prevalences were unexpectedly high, especially in children. This was probably related to a recent increase in transmission of filariasis as a result of extensive flooding in the area prior to the study. The study indicated that lymphatic filariasis is highly endemic in the Lower Shire area of Malawi, and calls for action towards its control.