Renal disease in chronic arthritis of childhood. A study of urinary N-acetyl-beta-glucosaminidase and beta 2-microglobulin excretion. Urinalyses of randomly obtained samples from children with various types of chronic arthritis revealed proteinuria in 2.3% of patients, hemoglobinuria in 3.5%, erythrocyturia in 4.1%, and leukocyturia in 5.3%; these frequencies are within the range found by screening school children. However, raised urinary levels of N-acetyl-beta-glucosaminidase and/or beta 2-microglobulin (both sensitive measures of renal tubular damage) were found more frequently in children with chronic arthritis than in controls (P less than 0.0001). Abnormalities of either N-acetyl-beta-glucosaminidase or beta 2-microglobulin excretion were associated with active arthritis as measured by physician global estimate of disease activity, with a polyarticular onset of juvenile rheumatoid arthritis, and with the use of slow-acting antirheumatic drugs or the concurrent use of more than 1 nonsteroidal antiinflamtory drug. Abnormal renal tubular function appears to be common in chronic arthritis of childhood. The long-term consequences of this abnormality remain to be elucidated. 