We studied the peak expiratory flow rate in residents of two villages in the Philippines; one with a high prevalence, one with an intermediate prevalence of microfilaremia. We also determined the presence and level of microfilaremia in the individuals by examining thick films of 40 cmm of peripheral blood taken at night. We have found that the slope of the peak expiratory flow with age is steeper in the village with a high prevalence of microfilaremia and that it is also steeper in individuals from the high prevalence village with microfilaremia as compared with those without parasitemia. We conclude that there is a relationship between the presence of microfilariae and airflow obstruction.