Because patients on intermittent catheterization often suffer asymptomatic bacteriuria it is important to know how serious such a colonization may be. We followed 156 hospitalized spinal cord injury subjects on intermittent catheterization. Daily bacterial cultures were performed. When bacteriuria was detected bacterial cultures and uncentrifuged urinary leukocytes were quantified. Gram-positive bacteria, such as Staphylococcus epidermidis and Streptococcus faecalis, produced minimal white cell response, even in high colony counts. However, gram-negative and fungal organisms elicited significant pyuria. Analysis of pyuria in conjunction with urine cultures may serve as a clinical guide to treatment of significant bacteriuria.