Urinary immunoglobulins, especially locally secreted IgA, are a measure of the immune response to urinary tract infection (UTI). Urinary immunoglobulins have been quantitated in normal children and children with glomerulonephritis, the nephrotic syndrome, and urinary tract infection. Children with urinary tract infection show a unique elevation of urinary secretory IgA. The highest levels of urinary secretory IgA were found in children with severe anatomic derangements. Secretory IgA was elevated after UTI in 36% of female children without an identifiable anatomic abnormality and 77% of those with an abnormality. Maturation of the local immune response of the urinary tract is evidenced by increasing urinary secretory IgA levels with age.