Foley catheters were introduced into 47 human subjects after spontaneously voided urine was collected and the amount of hematuria caused by catheterization itself was measured. A statistically significant increase in urinary erythrocyte count was demonstrated after catheterization, but the effect was small. All but one patient had fewer than four cells per 400-power field that were attributable to the procedure. Males demonstrated greater amounts of catheter-induced hematocyturia than females. Microhematuria greater than three erythrocytes per high-powered field should not be attributed to urethral catheterization. Because urinary specimens are often collected after catheterization of multiply injured patients, and the presence of hematuria may indicate serious genitourinary injury in such patients, knowledge of the contribution of catheterization to the total amount of hematuria should improve the diagnostic accuracy of the initial urinalysis.