The value of microscopy of a Gram-stained urethral smear in the diagnosis of non-gonococcal urethritis and chlamydial urethral infection was assessed in 153 men attending a department of genitourinary medicine. A mean of more than 4 polymorphs in 5 fields (x1000) was considered to be abnormal. In the diagnosis of urethritis, the sensitivity (94%), specificity (91%), positive predictive value (93%) and negative predictive value (93%) were high. In the diagnosis of chlamydial infection, sensitivity (91%) and negative predictive value (96%) were high, but specificity (68%) and positive predictive value (46%) were low. In men with symptoms of genitourinary infection, the sensitivity (96.7%) and negative predictive value (97.4%) of the Gram-stained urethral smear in the diagnosis of chlamydial infection was comparable with modern chlamydial antigen detection tests.