The observation of more than four polymorphonuclear cells (PMN) per high-power field (hpf) in gram-stained smears of urethral secretions was found to differentiate patients with urethritis from patients without urethritis. A urethral discharge was present in 78% of patients with nongonococcal urethritis (NGU). Dysuria without demonstrable urethral discharge and with fewer than four PMN/hpf did not appear to fit into the NGU spectrum. NGU is now defined to include men who have negative urethral cultures for Neisseria gonorrhoeae with a urethral discharge and/or more than four PMN/hpf in their urethral smears. The findings of more than four PMN/hpf in the urethral smears of 22%of asymptomatic sexually active men with more than one sexual partner (polygamous controls) suggests that asymptomatic NGU is not uncommon. Chlamydia trachomatis was isolated significantly more frequently from the NGU study group than from the control group (P less than 0.001). This study adds Corynebacterium vaginale (Haemophilus vaginalis), group B streptococci, and yeasts to the list of sexually transmitted microorganisms that are not etiologic determinants of NGU.