To determine the cause of the acute urethral syndrome, we studied 59 women with dysuria and frequent urination without "significant bacteriuria" (defined as greater than or equal to 10(5) organisms per milliliter), 35 women with typical cystitis and 66 women with no symptoms of urinary-tract infection. Although none of the 59 women with urethral syndrome had greater than 3.4 x 10(4) bacteria per milliliter in either of two successive midstream urine specimens, samples of bladder urine obtained by suprapubic aspiration or catheterization from 24 women contained coliforms, and samples from three contained Staphylococcus saprophyticus; all but one of these 27 women also had pyuria. Of the 32 women with sterile bladder urine, 10 of 16 with pyuria and one of 16 without pyuria were infected with Chlamydia trachomatis (P = 0.002). Chlamydial infection was found in 11 of 42 women with urethral syndrome and pyuria, in three of 66 without symptoms, and in one of 35 with cystitis (P less than 0.01 when the group with urethral syndrome is compared with either of the other groups). Thus, 42 of 59 women with urethral syndrome had abnormal pyuria and 37 of these 42 were infected with coliforms, S. saprophyticus, or C. trachomatis, whereas few women without pyuria had demonstrable infection. Bacteriuria of greater than or equal to 10(5) per milliliter may be an insensitive diagnostic criterion when applied to symptomatic lower-urinary-tract infection.