To determine the extent of lymphatic disease in persons infected with Wuchereria bancrofti who were microfilaraemic, we examined the superficial lymphatics of the legs by scintigraphy. In 4 endemic control subjects and in 10 of 14 clinically asymptomatic microfilaraemic individuals, lymphoscintigraphy revealed one major channel of lymphatic drainage in each leg. However, while trunk lymphatics were bilaterally symmetrical in the control, marked differences in the calibre of lymphatic vessels were observed in the microfilaraemic persons. Non-discrete lymphatics and a diffuse symmetrical distribution of collateral vessels in both legs were observed in all of 5 amicrofilaraemic patients with grade 2 lymphoedema. A similar diffuse drainage pattern was also seen in 3 previously microfilaraemic persons who had remained amicrofilaraemic and asymptomatic following treatment with diethylcarbamazine citrate (DEC). Thus, clearance of microfilaraemia by DEC therapy did not appear to reverse the type of lymphatic pathology observed in microfilaraemic subjects. The lymphoscintigraphy patterns did not correlate with serum levels of antibodies to 3 recombinant filarial antigens. Virtually all the asymptomatic microfilaraemic individuals infected with W. bancrofti examined had subclinical lymphatic disease detected by the non-invasive imaging technique of lymphoscintigraphy.