Microfilariae of O. volvulus were recovered from the urine of 11% of the residents of a village in the Republic of Chad where onchocerciasis was holoendemic. A follow-up study of the same population 3 years after the original investigation revealed that microfilaruria was still present and that living microfilariae of O. volvulus could be recovered from small urine samples obtained by high catherization of the ureters. Radiological changes consistent with chronic pyelitis were found in 4 out of 14 persons with microfilaruria who were examined by retrograde pyelography. The recovery of microfilariae in the urine was associated with the intensity of the infection, as determined by counts of microfilariae in skin snips and the number of onchocercomata. A systematic comparison for differences between onchocerciasis patients with and without microfilaruria revealed that the microfilaruria patients had a low weight: height ratio, deficient antibody responses in indirect haemagglutination tests with O. volvulus antigen, elevated serum aminotransferase levels, and reduced systolic blood pressure.