Percutaneous suprapubic urinary bladder aspirations were carried out in 654 infants and children. Five hundred of the aspirations were in patients admitted to the hospital with acute illness, and 154 were in patients being followed in the Pediatric Renal Clinic because of a history of recurrent infections of the urinary tract. In the latter group, a clean-voided sample of urine was obtained immediately after the suprapubic aspiration for a comparative bacteriologic study. In the first group of 500 patients, four unsuspected infections were uncovered, all of them in boys in whom a structural abnormality of the urinary system was demonstrated. These four boys represented 1.4% of the males in the group, giving an incidence of infection that is almost three times that shown in a study of boys in a school population. In the second group of 154 patients, three patients demonstrated a "bladder" bacteriuria in the presence of an indeterminate clean-voided bacteriuria. These patients were considered to have a low-grade bacteriuria and active, albeit suppressed infection. Thus, suprapubic bladder aspiration may be one means of resolving the problem of low bacterial counts in patients with chronic infection of the kidney and urinary tract. Percutaneous suprapubic aspiration of the urinary bladder is a safe, easy, and useful method of obtaining urine for accurate diagnosis of urinary infection. Its use as a routine procedure in all ill infants and children may reveal unsuspected infection. A comparative bacteriologic study of urine obtained by percutaneous aspiration of the bladder (vesicopuncture) and blood obtained by venipuncture may be fruitful and is presently under way.