A field survey of the house sparrow, Passer domesticus domesticus Linn., for the filarial nematode, Splendidofilaria passerina, was conducted in Illinois during May 1968 to May 1969. Fifty-four (19.9%) of 271 birds of both sexes were infected with adult male and/or female worms within the walls of the pulmonary arteries. A mean of 5.8 and range of 1–23 worms per bird were recovered. Forty (14.8%) sparrows were positive for microfilariae in smears of lung blood, but there was no correlation between adult worm burden and numbers of microfilariae per 100 microscopic fields. Eleven birds infected only with female worms were negative for microfilariae. Patent infections were first observed in immature sparrows during September and October, which suggests an approximate 3 month prepatent developmental period of the adult worm. The incidence of infection remained high throughout most of the year, with a decline during the months of June through August. The data indicate increased infection in the autumn, possibly related to the activity of an unknown vector during the summer months. The walls of the pulmonary arteries of heavily infected sparrows were grossly thickened and enlarged, with worms occupying most of the tissue and occasionally extending free into the lung and heart. Microscopic lesions included fibrosis, necrosis and stenosis of the pulmonary artery. Accumulations of foreign body giant cells were observed surrounding dead worms, but there were no reactive cells around living worms.