The purpose of this study was to explore the value of scrotal ultrasound as a means of evaluating Bancroftian filariasis. Color Doppler ultrasound examinations were performed to look for subclinical hydroceles and motile adult filarial worms (dancing worms) in dilated lymphatics. Sixty-one male subjects from a filariasis-endemic area in Egypt were studied including 19 clinically normal microfilaria (MF) carriers (seven with dancing worms and eight with subclinical hydroceles), 13 MF-negative subjects with positive filarial antigen test results (three with dancing worms and seven with subclinical hydroceles), 22 exposed subjects with no MF and negative antigen test results (no dancing worms, four subclinical hydroceles), and seven subjects with clinical filariasis (no dancing worms, seven hydroceles). Thus, all men tested with clinical filariasis and most clinically normal subjects with either microfilaremia or filarial antigenemia had abnormal ultrasound examination results. Ultrasound findings often changed after therapy with diethylcarbamazine, with disappearance of dancing worms and development of new scrotal calcifications or hydroceles. This study confirms the value of scrotal ultrasound as a means of noninvasively visualizing adult filarial worms and assessing subclinical lymphatic damage in Bancroftian filariasis.