Stool specimens were collected from 1995 sugarcane cutters on the Hippo Valley and Triangle sugar estates, Zimbabwe, in order to determine the prevalence and intensity of Schistosoma mansoni infection. Pathological changes normally ascribed to S. mansoni infection were assessed in the infected cutters, by ultrasonography before treatment. The height, weight, age, haemoglobin levels, blood pressure and body fat of the infected and uninfected control study subjects, standardized by age, were determined. Those with elevated blood pressure were excluded from the study. Physical fitness and work performances were assessed in 287 infected and 210 uninfected cane cutters aged (mean +/- S.D.) 36.5 +/- 7.5 years. Despite the finding that all the subjects were of good nutritional status and generally physically fit, a t-test showed a significant improvement (P < 0.01) in the performance of the infected cutters following treatment; age-related physical performance, measured by the Harvard Step Test, increased by 4.3% and work output, measured as the amount of cane cut in a given time, rose by 16.6%. Although the physical and work performances of the uninfected control subjects also increased over the same period, this trend, attributed to occupational physical exercise, was not statistically significant (P > 0.05). Significant correlations were found between both infection intensity-related pre-treatment physical fitness (P < 0.05) and work performance (P < 0.01) and prevalence of Symmers' peri-portal fibrosis.