The most common cancer in Egypt is urinary-bladder carcinoma, which is strongly associated with infection with Schistosoma haematobium. The object of this study was to determine whether infection with this parasite resulted in an increase in chromosomal breakage in the urothelium. Urine samples were collected from 37 Egyptian farmers with S. haematobium ova in their urine and from 32 non-infected males attending the same village clinic. The urine was centrifuged and the urothelial cells present in the sediment were assayed for the presence of micronuclei, a quantitative indicator of chromosomal breakage. Micronucleus frequencies were significantly higher in the infected group (mean frequency, 0.97 +/- 0.12%) than among controls (mean frequency, 0.12 +/- 0.04%, p less than 0.001). There was a trend towards an increase in micronucleus frequencies in infected individuals who smoked (10 of the 37 subjects) compared with those who were non-smokers, although this effect was not significant (p = 0.12). Bladder infection was associated with a 6.8-fold increase in micronucleus frequencies among non-smokers. In a pilot study, 11 patients were treated with praziquantel, an anti-schistosomal agent. Mean micronucleus frequencies were reduced significantly (before treatment, 1.27 +/- 0.16%; after treatment, 0.30 +/- 0.16%, p less than 0.001), thus supporting a direct involvement of the infection in increased chromosomal breakage in the urothelium.