Although sexually active female adolescents are often routinely screened for sexually transmitted diseases, indications for culturing adolescent males for sexually transmitted urethral infections are controversial. A study of 54 sexually active males (14 to 22 years old) was undertaken to assess the reliability of using dipstick leukocyte esterase activity in first-catch urine specimens to detect urethritis caused by Neisseria gonorrhoeae and Chlamydia trachomatis. Eighteen males had sexually transmitted diseases: N gonorrhoeae (nine patients), C trachomatis (eight patients), and N gonorrhoeae and C trachomatis (one patient). First-catch urine specimens with a 1+ (mild) or 2+ (moderate) reaction on dipstick testing had a leukocyte (WBC) count of 10 WBCs per high-power field or greater on microscopic analysis, with an 83% sensitivity, 100% specificity, a 100% positive predictive value, and a 92% negative predictive value for the presence of N gonorrhoeae, C trachomatis, or both. Clinical criteria for screening (urethral discharge, dysuria, or exposure to a sexually transmitted infection) plus a dipstick-positive first-catch urine specimen had a 94% sensitivity, 89% specificity, an 81% positive predictive value, and a 97% negative predictive value.