Microscopic examination of the urinary sediment to determine the degree of pyuria is an accepted method to screen for urinary tract infection. We investigated the significance of pyuria in relation to the method of specimen acquistion, number of white blood cells and isolation of pathogens on culture. Only 36 per cent of our patients with more than 10 white blood cells per high power field on examination of the first random specimen had more than 10 white blood cells per high power field when a repeat clean catch midstream specimen was examined, and only 20 per cent of the patients had more than 10(5) pathogens per ml. on culture. However, the finding of more than 10 white blood cells per high power field on a clean catch mid stream specimen indicated more than 10(5) pathogens per ml. in 40 per cent of the cases. The use of a higher threshold for significant pyuria (more than 20 white blood cells per high power field) on examination of a random specimen increased the incidence of more than 10(5) bacteriuria found in specimens with initial pyuria by 43 per cent and represents the additional detection of 8.7 per cent of the total poputation studied. These data indicate that when screening for pyuria and infection one should either obtain a clean catch midstream specimen for examination of urinary sediment or increase the threshold for significant pyuria on a random specimen.