This year-round case-control study investigated treatment costs and work time loss to people affected by chronic lymphatic filariasis in two rural communities in south India. About three-quarters of the patients sought treatment for filariasis at least once and 52% of them paid for treatment, incurring a mean annual expenditure of Rs. 72 (US $2.1; range Rs. 0-1360 (US $39.0)). Doctor's fees and medicines constituted 57% and 23% of treatment costs. The proportion of people seeking treatment was smaller and treatment costs constituted a higher proportion of household income in lower income groups. Most patients did not leave work, but spent only 4.36+/-3.41 h per day on economic activity compared to 5.25+/-3.52 h worked by controls; the mean difference of 0.89+/-4.20 h per day was highly significant (P<0.01). This loss of work time is perpetual, as chronic disease manifestations are mostly irreversible. An estimated 8% of potential male labour input is lost due to the disease. Regression analyses revealed that lymphatic filariasis has a significant effect on work time allotted to economic activity (P<0.05) but not on absenteeism from work (P>0.05). Female patients spent 0.31+/-1.42 h less on domestic activity compared to their matched controls (P<0.05). The results clearly show that the chronic form of lymphatic filariasis inflicts a considerable economic burden on affected individuals.