In a prospective study of urinary tract infection in 57 boys, diagnosis in 50 patients was based on the presence of more than 10(5) organisms per milliliter of urine. Gram-positive cocci were more prevalent, and Escherichia coli was less prevalent than previously reported in girls. Fever and urinary tract abnormalities were frequent in boys younger than 10 years of age; less common in boys 10 to 14 years old. Excretory urography and voiding cystourethrography were performed in 30 boys. In 15, one or both showed abnormalities, usually vesicoureteral reflux. Most of the abnormalities, and all of those that necessitated surgical correction, were in boys under 10 years old. In the remaining seven patients, less than 10(3) organisms per milliliter grew on urine culture. All of the seven were younger than 10 years of age, and all had serious renal abnormalities, three requiring surgical correction.