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First-catch urine samples of 130 men attending a clinic for genitourinary medicine were screened with the leukocyte esterase dipstick test. Conventional microscopy of centrifuged sediment and inverted microscopy of unspun urine samples were performed 2-4 hr later. The leukocyte esterase test had sensitivities of 100% and 96.6% and specificities of 55% and 52.8%, respectively, when compared with the two microscopic methods. The apparently low predictive value of the positive leukocyte esterase test was probably linked to its ability to detect lysed cells and the limitations of the microscopic methods, which were used as the absolute indicator of nongonococcal urethritis. Of the 42 patients with microscopic pyuria, 45.2% had positive cultures for Chlamydia trachomatis, and the leukocyte esterase test was positive in every case. Twenty-one per cent of the C. trachomatis-positive patients were asymptomatic, and 42% had fewer than five polymorphonuclear leukocytes per high-power field (X1000) of the gram-stained urethral smear. The leukocyte esterase test appears to be a simple, practical, and sensitive test that is useful in screening for nongonococcal urethritis, especially asymptomatic nongonococcal urethritis.