Microfilariae of Wuchereria bancrofti were observed in cytologic material in 35 cases. The material included cervicovaginal smears (17 cases), effusions (14), urine (2), bronchial washings (1) and ovarian cyst fluid (1). The initial diagnosis was made from the cytologic smear in all cases; none had clinical filariasis. Symptomatic vaginal bleeding in 9 of the 17 cases with microfilaria-positive cervicovaginal smears was reflected in the large numbers of red blood cells found in the smear. Blood eosinophilia was present in 11 of 19 cases investigated. Eosinophils were seen in the smears in 20 cases. In the majority of the cases of effusions with microfilariae the effusions were malignant. Significant adherence of inflammatory cells and macrophages to microfilariae was present in 7 of the 35 cases. The significance of these findings is discussed.