A study was carried out in the Kingdom of Tonga, an area of hyperendemic Bancroftian filariasis, to determine whether correlations could be made between microfilaraemia, as diagnosed by membrane filter concentration, and immunological (skin test, immunoglobulin levels) or clinical findings. There was no relationship between the presence or degree of microfilaraemia and any clinical manifestation or skin test reaction. The skin test positivity rate for microfilaraemic and amicrofilaraemic individuals was approximately the same for all age groups. Among those aged 0 to 4 years, 48% of microfilaria positives were negative in the skin test. The highest average IgG and IgE levels were found in the groups with the highest microfilarial densities, i.e., in children with a history of fever and in adults with a history of lymphangitis/lymphadenitis. Over a period of a year, the microfilarial density changed significantly in 18 (34%) of 53 adults.