This paper reports the results of 10 years of observations on the clinical manifestations, pathology and immunity to filariasis and aetiological biology of filariae in three volunteers (first author and his family members) who were inoculated experimentally with infective larvae of periodic Brugia malayi in 1981. The changes in clinical symptoms and signs were recorded systematically. Microfilariae were first detected at 41 and 46 weeks after inoculation in two subjects and remained detectable in small numbers until 8-8.5 years after infection. The microfilarial density fluctuated at 1-2 mf 120 microliters-1. Thereafter no microfilariae were detected in 12 blood sample examinations, suggesting that the adult reproductive period of periodic B. malayi could last up to 8-9 years in the human body. Eosinophilia occurred mainly before and at the initial stage of microfilaraemia. An increase in the lymphocytes was observed to some extent at 2-156 weeks after infection. Biopsy at the inoculation site 6 weeks after inoculation showed infiltration of the lymph node by inflammatory cells, mainly eosinocytes, lymphocytes and monocytes. Lymphangiectasis and lymphostasis were observed in both limbs and pelvic regions by lymphangiogram taken 11 weeks after inoculation. Antibodies against B. malayi first appeared at 2-5 weeks after infection, peaked at 12-56 weeks and thereafter declined gradually. Subjects A and C became antibody free but subject B remained positive to antibody against B. malayi 10 years after infection. E-rosette forming lymphocytes became lower than normal at 11 weeks and recovered to normal within 10 years after infection.