We described the results of ten-year observations on the clinical manifestations, pathology, 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 and without use of any antifilariasis drugs. Microfilariae were first detected at 41 and 46 weeks after inoculation in two subjects and remained detectable in a small number until 8 and 8.5 years after infection. Eosinophilia occurred mainly before and at the initial stage of microfilaremia. The increase in the lymphocytes to some extent was observed at 2-156 weeks after infection. Biopsy at inoculation site 6 weeks after inoculation showed the lymph node with the infiltration of inflammatory cells, mainly eosinocytes, lymphocytes and monocytes. Lymphangiotasis and lymphostasis were observed in both limbs and pelvic regions by lymphangiogram taken 11 weeks after inoculation. Antibody against B. malayi first appeared at 2-5 weeks after infection and reached the highest at 12-56 weeks and thereafter declined gradually. Two subjects turned to be antibody free 10 years after infection. E-rosette forming lymphocytes became lower than normal at 11 weeks and recovered to normal within 10 years after infection.