Guatemalan volunteers with onchocerciasis were given a dose of diethylcarbamazine to learn if this caused migration of microfilariae of Onchocerca volvulus into urine, blood, and sputum. In 5 of the 10 volunteers, the numbers of microfilariae in the urine increased considerably following the drug. In the same 5, relatively large numbers of microfilariae were observed in the blood and sputum specimens. Response in the remaining 5 was negligible. Four controls given a placebo did not respond. Interestingly, however, 2 control subjects had onchocercal microfilariae in their concentrated blood specimens and 3 control subjects had microfilariae in the sputum. Therefore, we believe that microfilariae, if searched for, may be found in other parts of the patient not normally associated with the infection.