The intensity of infection with Schistosoma haematobium (worm burden) can be approximated by quantifying the number of parasite eggs excreted in the urine. A new method of egg counting has been developed in which urine samples are passed through transparent Nuclepore filters (Nuclepore Corp., Pleasanton, Calif.). The method requires no staining, is rapid, and can be performed in the field. It has previously been reported, however, that there is a significant daily fluctuation in output of eggs in the urine of patients with schistosomiasis haematobia. In the present study, daily urine samples were obtained on each of the 10 school days during a 12-day period from 121 schoolchildren in Coast Province, Kenya (total, 10 samples per child). In terms of detecting infection, it was of interest that egg counts of 0 were very rarely recorded for subjects whose mean egg counts were greater than 20/10 ml of urine. When the patients were classified into one of four groups, according to egg count (less than 1, 1--100, 101--400, greater than 400), the proportion correctly classified based on a single urine specimen was 75%; this proportion rose to 78% when the classification was based on two specimens and to 83% with three specimens. Most of the urine specimens that were misclassified belonged in an adjacent group.